===== SOLUTION =====
The scaling problem that prompted this thread is of general interest and is solved here: https://www.gog.com/forum/myst_series/residualvm_apparently_has_hidden_parameters.
Also see https://answers.microsoft.com/en-us/windows/forum/games_windows_10/solved-how-can-i-get-change-high-dpi-settings-to/f073795f-8500-4b60-b76f-e838e7ba7227?tm=1536683457680
=====

Game is Myst 3.

OBJECTIVE: A 1280x720 game image (with well-sized 'finger') that Windows then scales up to 3840x2160.

WHY IMPORTANT: If I set these:
'fullscreen=true'
'fullscreen_res=desktop'
then the game image is 3840x2160 -- Yeah! -- which Windows doesn't need to scale up -- Yeah! -- but the 'finger' (i.e., the mouse pointer) is infinitesimal -- almost impossible to see.

I achieved my OBJECTIVE just one time, then it broke. Since then, Windows no longer scales the 1280x720 up to 3840x2160 but just black-mattes it to 3840x2160 (with infinitesimal 'finger').

Here's my myst3.ini:

[residualvm]
gui_saveload_chooser=list
filtering=false
autosave_period=300
mute=false
speech_volume=223
aspect_ratio=true
talkspeed=179
renderer=opengl_shaders
gui_browser_show_hidden=false
subtitles=false
fullscreen=false
console=false
sfx_volume=118
music_volume=76
speech_mute=false
lastselectedgame=myst3-win
vsync=true
kbdmouse_speed=3
versioninfo=0.3.1
gui_theme=MODERN

[myst3-win]
filtering=true
description=Myst III Exile (DVD/Windows/English)
widescreen_mod=true
savepath=C:\GOG Games\Myst 3\SAVEGAME
renderer=opengl_shaders
path=C:\GOG Games\Myst 3
fullscreen=true
gameid=myst3
language=en
fullscreen_res=1280x720
platform=windows
vsync=true
aspect_ratio=true
guioptions=sndNoSubs sndNoSpeech sndNoSFX sndNoMIDI gameOption1 lang_English

Windows is making Registry entries that control scaling, but they are binary settings and I've not found any documentation regarding how to interpret the binary.

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Compatibility Assistant\Store]
"C:\\GOG Games\\Myst 3\\residualvm\\residualvm.exe"=hex:53,41,43,50,01,00,00,\
00,00,00,00,00,07,00,00,00,28,00,00,00,0e,20,9c,00,92,0e,9d,00,01,00,00,00,\
00,00,00,00,00,00,00,0a,61,20,00,00,bf,a2,13,9d,ed,d1,d3,01,00,00,00,00,00,\
00,00,00,05,00,00,00,10,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,\
00,80,02,00,00,00,68,01,00,00,00,00,00,00,10,00,00,20,00,00,00,00,00,00,00,\
00,00,00,00,00,00,00,00,00,63,11,84,00,00,00,00,00,3c,00,00,00,1b,00,00,00,\
00,00,00,00,00,00,00,00,00,00,00,00,01,00,00,00,00,00,00,00,00,00,00,00,53,\
af,05,00,00,00,00,00,1d,00,00,00,00,00,00,00,00,00,00,00,00,00,00,a0,00,00,\
00,00,00,00,00,00,00,00,00,00,00,00,00,00,78,7e,1a,00,00,00,00,00,23,00,00,\
00,00,00,00,00,00,00,00,00,10,00,00,a0,00,00,00,00,00,00,00,00,00,00,00,00,\
00,00,00,00,1a,4f,00,00,00,00,00,00,07,00,00,00,00,00,00,00,00,00,00,00,10,\
00,00,a2,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,ee,32,02,00,00,00,\
00,00,07,00,00,00,00,00,00,00,00,00,00,00,12,00,00,a0,00,00,00,00,00,00,00,\
00,00,00,00,00,00,00,00,00,6d,22,00,00,00,00,00,00,01,00,00,00,00,00,00,00,\
00,00,00,00,00,00,00,22,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,8c,\
22,00,00,00,00,00,00,01,00,00,00,00,00,00,00,00,00,00,00,00,00,00,a2,00,00,\
00,00,00,00,00,00,00,00,00,00,00,00,00,00,70,3e,00,00,00,00,00,00,02,00,00,\
00,00,00,00,00,00,00,00,00,00,00,00,e0,00,00,00,00,00,00,00,00,00,00,00,00,\
00,00,00,00,6b,1d,00,00,00,00,00,00,01,00,00,00,00,00,00,00

Can anyone here lend a hand?

As things currently sit, the game is unplayable.

Warm Regards,
Mark.

I'd simply try using ResidualVM.

I am using ResidualVM.

Shame on me: I hadn't seen any reference to ResidualVM in your post and I didn't know if the GOG release came prepackaged with ResidualVM or not.

Question: Why is your resolution so high in the first place?

In general or related to Myst3?
General:
Probably because it adds compfort. Sharper desktop (text benefits a hell lot, just look at current smartphones) at same size. Even if you are playing DOS-games you do benefit, course the NearestNeighbourScaling leads better results (the pixel blocks are more refined due to smaller subpixels)
Myst3? Maybe the text is stored in fonts, it then could be very pleasent to read in UHD. In general, scaling the rest of the image should lead to better results then lower resolutions.

Sorry I took so long to reply -- I'm not getting email notifications from the forum software, so I didn't know there was new content.

My laptop's built-in display is 3840x2160. Running a game in fullscreen means scaling it up to 3840x2160. Correct me if I'm wrong. Running a game in a 1280x720 window on a 3840x2160 screen is like trying to write Magna Carta on a postage stamp. The only way the game is playable is to run it fullscreen. Fullscreen is 3840x2160.

No, you aren´t wrong, this is exactly one of the major points. But maybe you´ve missed that my posting was more or less a general statement, as I don´t know a lot about Myst3.

Trying to throw in some help: Have you tried GeDoSaTo? That tool can scale a hell lot of stuff and although its major focus is Downsampling, you can also Upscale a 1280x720-game with it. Give it a try!

A apologize again for delay... not getting notifications... I think I misunderstood your comment. You see, if I tell ResidualVM (the game execution wrapper) to render the game at 3840x2160, it does it, and it does fill the screen, but ResidualVM fails to scale up the mouse pointer ('finger'). The result is the mouse pointer can't be seen -- too small, 50% transparent. However, if I tell ResidualVM to render the game at 1280x720 and let Windows scale it up, the game still looks great and the larger mouse pointer (which also got scaled up by Windows) makes it playable. I hope I'm being clear. Good tip for some. In my case, the Intel display driver does that automatically.

Check your Intel driver settings and try dropping the Windows desktop resolution to something much lower, like 1080P, before you run the game. That way, when the game runs "full screen" it will run on the desktop setting of 1080P. Your cursor should be a lot bigger, where at least you can see it. If not, drop the desktop res down a bit more, until you are happy. The graphics won't suffer because of reasons already mentioned in this thread--the game is rendered and scaled up from 640x480.

I would never recommend someone with a small laptop screen to purchase a 4k monitor with it simply because of the issue you have just outlined. I have a 32" 4k 3840x2160 .18 dot pitch monitor, and at full 4k my cursor in Myst III is barely visible--but at least I can see it. I can imagine that yours might not be visible at all...! I actually think even 27" of monitor is not enough for 4k res, but that's just me...;)

Good luck! that first tip should do it for you.