after careful discussion I have come to the conclusion it is not feasible to get the migration done in time for 3.2 final, especially considering that December is not exactly a time where everyone wants to be doing extra work.
All releases up and including 3.2 will therefore be made from SVN. 3.2 will face the same small issue as 2.7 and 3.1, i.e. that subsequent releases will have different-looking build identification.
Dirkjan is positive that he will have the test repo ready by the end of the month. That means that if the issues left in the test repo are small enough to be done in a few days, SVN will be frozen as soon as 3.2 final is out of the door, and anything committed to py3k from then on will go to Mercurial once it's ready.
I hope this is a good compromise for everyone.