GTK peers on OpenJDK
With surprisingly little change on OpenJDK code itself, and some tweaks in the GTK peers (to account for some new methods in the peer interfaces) I was able to get the Classpath AWT demo running quite nicely on with GTK peers on OpenJDK class library with JamaicaVM:
Now on to figuring out how to do similar with the graphics stack. This will be a harder battle I suppose.