With jamvm-1.5.0 and a couple of eclass patches it's now possible to 
emerge things with jamvm as a JDK. You do get a warning about tools.jar 
missing so we have to think about what to do with that.

pena commons-net # devebuild commons-net-1.4.1-r1.ebuild clean compile
  * commons-net-1.4.1-src.tar.gz RMD160 SHA1 SHA256 size ;-) ... 
 
                                                          [ ok ]
  * Using: jamvm

BUILD SUCCESSFUL
Total time: 3 seconds
 >>> Source compiled.

Regards,
Petteri