Sounds cool and I like the idea. But:
  1. A bit misleading to announce "No special software needed" when the user will actually have to download a language model. I get it that the file will stay in cache and from the user point of view they only see their browser, but still...
  2. You'll probably need to provide a few more explanations (and guarantees) before convincing people to jump in and see their GPU working 24/7. How will it affect the overall computer performance? What about heating? Can you restraint the computing power you share? Are there any data files stored on the computer somewhere (with possible legal issues...)?
Anyway, good luck. This thing could be big.