I finally managed to upload the modified jbullet source as its used in jME3 to github:
I had a checkout of the original jbullet source from here: http://jbullet.advel.cz as well and synced the updates to that in irregular intervals manually by creating a diff to “our” source and applying the changes. The last sync like that happened over two years ago though - I don’t know if there were any changes to the original jbullet in that time though as its development seemed to slow down more and more.
Most of the changes I made to the source are “official” fixes and additions, like that for the maximum force for the vehicle or the heightmap collision shape. But the jbullet developer never included my patches (except for the heightmap I think). Other changes are ghost vs. character collisions and a pre tick callback, which also exists in the native bullet.
All changes that I made should be marked by a “normen” comment in the source.
Hope this helps,