To this US'n ear "used not to..." doesn't sound right. "Used to not..." sounds better despite the split infinitive but I would probably use "didn't used to...".

Regarding the member of the gonna-wanna-hafta group I have seen useta. It still has some problems with that e in there ( in the same vein I useta pronounce oncet wun-set) but, in the words of the noted philologist, "Wutcha gunna do?"