FWIW, I thought that you acknowledged that this problem was reported in #299009 (over 25 months ago). I'm mentioning this because I believe this issue is/was the main reason for "anti-aptitudism".Apparently there have been bugs in this for years and no-one reported them until they caused trouble for the d-i team several months ago. They should be fixed in stable's aptitude now, and I would appreciate bug reports on any transition problems that remain.
Anyway, if it's resolved, I'll have to give aptitude a try. I second Michael in thanking you for "bringing this feature at the right place".