Hi there,
A simple question - can I use FS2_Open (i.e. through the FS2_Open installer available at Hard Light Productions) with the original Freespace 1/Silent Threat copy available here, or is it only usable with Freespace 2?

The instructions say to copy the .MVE files over from the cd's data/ directory, in this case, it's data1 and data2 instead of 2 different CDs. Then you run the FS1 mod and it starts up freespace.
Though I can only start the campaign and it (the mod script) crashes on me. So I can only half answer your question...

FS2_Open is solely usable with FS2, yes, but you can use the Freespace Port on HLP to play FS1 in the FS2_Open engine.

My understanding is that FS2_Open only works with FS2, but you can download the Freespace Port as the poster above me said, and then move the .MVE files over. The .MVE files are the cutscene movie files and FS2_Open now supports playing them directly (you used to have to convert to .avi and fiddle with them)

Yes, it's possible to play Freespace 1 with mediaVP in FS2Open with FSPort by now. Probably wasn't available when the thread was made.