I see JDBM is being dropped and this is covered in the FAQ.
But JDBM is still being used for the Apache Directory and it seems currently it's been adopted into their source tree: http://svn.apache.org/viewvc/directory/apacheds/trunk/jdbm/
There are fixes going in every so often: http://jdbm.cvs.sourceforge.net/viewvc/jdbm/jdbm/src/main/jdbm/btree/BTree.java?r1=1.17&r2=1.18
So I'd be happy to port it if you would reconsider interest in having it...
It seems hashing/b-tree database projects don't really have too many releases. GDBM, as an example, was last released in 2002.
Sure, if you are interested in porting and maintaining a module for JDBM, why not. Have a look at the existing cache stores and their tests for details on this. If you haven't already joined the infinispan-dev mail list, I'd encourage you to do so.