As a consumer, if I were to spend money on a game via whatever distribution channel I prefer (e.g. Steam, HIB, USC, etc.), I expect it to work out of the box, e.g. I download an installer from the distributor, I run it, and voila, I'm ready to play (in the USC's case, I open it up, click a few buttons, deb is installed, and I can run it right away). I'm not too keen on paying for a binary file, and then realizing that I have to pay for and download the game's data separately, and then move the game data into the same folder, fiddle with permissions, etc. At that point (assuming I'm your average consumer), I would've just given up, asked for my money back, and moved on to another game.
The only people who have the rights to sell a complete game bundle for Aquaria (with engine and all game data bundled together) are Alex and Derek, so it's up to them whether they choose to support the USC or not. IMHO, selling a GPL'ed game binary (without data) is pointless; either the end-user has no idea why the game doesn't work out of the box and is reluctant to invest any more effort to get it working, or the end-user knows that the engine is GPL'ed and source is available, and they can compile it themselves.
(I'm not against having Aquaria on the USC; it's just that I, personally, wouldn't bother putting time and effort into making it happen, unless Alex and Derek also express interest in uploading Aquaria to the USC.)