Hey,
I'll give this some thought and draft around a little.
I don't want to disappoint you in any way with my view on the google bit, neither is it my aim to discredit your approach - but since I have a very different toolbox from what google offers at hand, going that direction is not an option to me.
Instead of discussing the pros and cons of the g-service, I'd rather try to focus on creating the most inviting page out there to engage in mixing comparisons/contests.
Most of that can probably achieved by puzzling together the already existing wikiloops bits and pieces in a new way.
One of the major issues you are pointing at which will probably be the "thrive or die"-factor to such a tool is the need to be able to spot wikiloops collaborations offering ALL single tracks before one starts a remix contest (I don't know why the word "contest" seems best to me in this context, don't let that confuse you, I'm brainstorming).
So, to me, a first thing to create is a list of wikiloops collabs which do have all single tracks. No big deal to do that, but crucial to know where one can start off.
Next, one would need to be able to pick one of those tracks and start a new remix contest, thus creating a new page on which the comparison will happen (that's pretty much what you worked on, as far as I have seen).
Then, that newly set up "contest" needs to be promoted in some way (think: "latest remix challenge" on the wikiloops home page) to attract some interested sound engineers - the best comparison page won't help if you can't attract people to visit it :)
I'll do a visual draft of which functionalities I'd like to include on a sample comparison page later on and post that up for your review, like I said, I believe most of the desireable functions (comments, players, newsfeed alerts on interactions, easy-upload-link etc) can be adopted from the other wikiloops pages quite easily.
what I would not want to mix up is the act of adding ( = recording something on top) and remixing ( = remixing a given set of audio tracks), so offering a fixed set of tracks to use right on the comparison page seems the ideal solution here.
Uploads to a mixing contest could show up in the "normal" wikiloops as "Mixer" remixes, they really only need one additional parameter (the ID of the contest, so to speak) to make sure they appear on the correct comparison page, too...
shouldn't be too much work to puzzle that up, I'll see what can be done :)