Are there any major disadvantages with using a wrapper for the collections to get a nice interface?
This is what I mean:

Official, native support for generics in NHibernate does not exist yet - work has just started. In the mean time, I'm using this: http://www.ayende.com/projects/nhibernate-query-analyzer/generics.aspx Is that what you're looking for?

Thanks for your answer!
That package seem to provide generic collections, but also do some additional stuff on bidirectional mapping. I'm a little bit reluctant to using it because of that. I want my quick-fix for using generics to be as easy to remove as possible when the generics support become native in NHibernate.

So, are there any problems doing it my way?

no, if you map with a field strategy.

go for it!

It does seem like a lot of work though, you could also use ayende's solution and rip out the bi-directional stuff.
Or you could look at the cvs, i think that mike is checking in several generic bag related files as we speak. So it will be a matter of days before you have something beta-like to play with.