Because it's all talk until the code runs, I have finally imported my code into a Google Code project (http://code.google.com/p/activehibernate). There is no formal release yet, so you'll have to use a Subversion client to get it (or just browse the repository online). There's a readme file that explains how to get the tests running.
All feedback is more than welcome, as I'm new to (J)Ruby. I'm also very interested in ideas for a Ruby-like interface to Hibernate. The first thing that should be done is replacing the hbm.xml mapping files by a kind of annotations or a DSL (inspired by Rails or GORM, the OR/M in Grails).