My quick read is that this looks very good, Lars. If you've missed something minor I'm sure it will be obvious when you're implementing it but I don't think you've missed anything big.
Probably need to do some error checking, i.e. if someone's named a post-instantiation proc that's not the magic name yet one of the magic name also exists you probably don't want to blithely continue on but to should give an error message and crap out.
I agree this doesn't need to be fancy, though - some simple sanity checking and an explanation as to why the package install and mount failed is sufficient. Presumably we won't release packages to users in such a broken state ...
