New in trunk. Please give this a try:
This is not the same many2many as Django and Rails do it (because does
not use an intermediate table) but for practical purposes it works the
same, if not better, and works on GAE too. It will possible to change
the widget to make it look different.
The trick is to turn a reference into a 'text' field and set the
IS_IN_DB or IS_IN_SET validator to multiple=True.
If you like it and if nothing breaks, this will go in 1.55.
Mind that I had to make quite a lot of changes to make this work