I've added some scores to the list on the main page. Separating out the scores and soundtrack sounds like a good idea, but we're running out of space in the thumbnail release listing area...
I like the idea of moving HTDA somewhere else to free up space and make it look a little cleaner. Maybe replacing the Year Zero ARG box with something like "Related Projects" like you said. What else could go under there, besides HTDA and Sonoio? Would we include Filter? Tapeworm? Pigface? If we don't have enough, we could probably expand our definition a little and include APC, Manson, Tool, etc.
The discography on the official site has a "Family" and "Miscellaneous" section. In my opinion, if we're adding something with thumbnails, it should be closer to "Family". HTDA should be its own section; ideally, so should Atticus Ross since he's an official member. SONOIO, 12 Rounds, Cortini, Telefon Tel Aviv, and New Regime, like it is on the official site makes sense.
The problem with "Related" is it opens too many doors, then you're stuck highlighting only a few with thumbnails, which is fine but not ideal. If you're going to throw up APC for writing credits, then why not Lil Nas X? If you're putting up APC because of touring as well... I'm sure you can see where that's going. If you want to have a section like that then I'd highlight the biggies, such as Manson, Pigface, Saul Williams, David Bowie... I had to stop myself there.
That's a lot to say my vote is for Atticus Ross, first and foremost.