In a surprising turn of events, I think I found the missing piece. Based on information found in this post, I changed ffmpeg.lib back to ffmpeg and added the following line to my installPhase:
patchelf --add-needed libavcodec.so $out/splashtop-business
What I noticed previously, despite my inclusion of ffmpeg is that this line: libavcodec.so -> found: /nix/store/3zizvz8c3ngcpnf3waw247x56mka7fph-ffmpeg-6.0-lib/lib wasn’t appearing when the derivation built. It seems like most of the dependencies are required when the application starts up but libavcodec.so is probably dynamically loaded right before a remote desktop session.
And with that… it worked! I was able to connect to a working remote desktop session!
I also went ahead and changed ffmpeg back to ffmpeg.lib and rebuilt just to see if it would continue to work with a more specific output scope and it’s still working!
Obviously, I still need to add in the ability for this derivation to fetch the tarball right from Splashtop and it also needs to create an application shortcut for launching the app graphically, but it’s close!
Only “bug” I’m noticing is that every login acts like it’s the first and I believe it’s because the program is unable to write logged-in user specific configuration that it uses on future app loads. In short, it just doesn’t remember my username, password, or 2FA between launches. Not even a deal breaker, but something I’d probably like to find a fix for.