My first attempt in using generic collections in NHibernate was to just make NHibernate grok my custom collection. I took a look in the code, and it seemed rather straightforward to do so. I posted to the developer list and got the following response from Sergey, the lead developer:
It's straightforward but a long process, and it involves some deep knowledge about the way NHibernate works. Luckily I managed to get it most of it pre-baked by transforming the existing Set collections to use generics. I started to run into problems when I had to dynamically construct generic types, but this article sums it pretty well. I could get the correct info and get it to work.
I decided to wait with that for now, and try a less invasive approach. I'm currently trying to get it to work using a Property Accessors. I'll post my results when I'm done.