Presumably they would test Hermione out on a single dementor first. Harry made that point about Voldemort testing horcruxes, so presumably he would do that as well. Also, didn't Voldemort write down the directions for resurrecting Hermione in case something went wrong with her Horcrux?

But your point about the horcrux likely not restoring magic lost through killing dementors seems a good one. I'd consider that likely. But if she could get her magic restored, that would open up a lot of ritual magic to her.