If you want to work on this bigger ORM refactoring I'm available for mentoring help with the App Engine backend (most probably others, too) and with non-relational DB concepts in general (if you need/want my help there). Like I said, we initially wanted to do something along the lines you suggest, but we couldn't really justify the significantly increased amount of work compared to the current approach. If you think it's worth the effort then please go for it.

In any case, the final nonrel backend API won't be much (or any) different from what we have now, so if someone wants to write a backend, please do so.