I wonder if it would be possible to get rid of the interlacing by a simple line doubling shader implemented in Daum's DOSBox SVN builds (there is a folder with shaders in plaintext that seem to be compiled at runtime if i understand this correctly).

http://ykhwong.x-y.net/

Two very quick and dirty (and admittedly unsophisticated) ways to implement this:

Check if a line is completely black and if so, replace it with the pixel color values from the line above. True this might give an unwanted additional line here and there, but I think that would be barely noticeable in this particular game that consists 99% of FMV.

If that is too computationally expensive, just check if a Pixel is black and then simply replace it with the color of the pixel directly above. True, that would give way more unwanted artifacts, but there are very few static scenes and screens in this game where it would matter.

Does anyone have tips how to implement that in the shader language employed by Daum's build? Or maybe there already is a more sophisticated interlacing removal shader out there (unfortunately it's not the same thing as deinterlacing for TV video signals).

Those lines are part of the video, I don't think you can change it

You could try this:

https://www.gog.com/forum/phantasmagoria_series/workaround_for_black_scanlines_in_phantasmagoria_2

I just tried this and it works.

Here's a few comparison pictures:
http://imgur.com/a/9TwCD

Also, it's good to use the SVN Daum version of DOSBOX because it supports save states, so you don't have to play the entire game in one go.

Didn't work for me. Which config file did you change? There's one in the main folder (dosbox_d.txt) and another one in the DOSBOX directory (dosbox.txt). I did add that line for the shader, but it didn't work! :( HELP!

There are two config files. One in the main game folder and another in the DOSBOX folder. If you are using the default file structure in Win 7 the main folder file is:

GOG Games/D - The Game/dosbox_d.conf

there is another config file in:

GOG Games/D - The Game/DOSBOX/dosbox.conf

Changing the config file in the main directory enabled the "fix" for me, however on my laptop it becomes so dark it is almost unplayable and I think I will play it with the shader fix rolled back but using Daum's Dosbox for the save states.

Does anyone know if there is a brightness option in Daum's Dosbox? I can't find one.