4.2pre8 does start and run, but it barfs on several plugins and posts a big old error on startup iff BeanShell 2.0b1 is in the standard Java Extensions folder. So it looks like for the moment the only solution is to manually add BeanShell to the classpath whenever I need it outside jEdit. Not really satisfactory.

Two more solutions:
1. copy jedit.jar from jEdit.app/Contents/Java/jedit.jar to /Library/Java/Extensions/
+ bsh works
+ jEdit works
- MacOS plugin dies
2. extract bsh-classes from jedit.jar, repackage to a new jar and copy that to /Library/Java/Extensions/ - i.e.
% jar -xf jedit.jar bsh (while in jEdit.app/Contents/Java/)
% jar -cf bsh-2.0b1-jedit-1.jar bsh
% sudo cp bsh*jar /Library/Java/Extensions/
+ all works
- bit more work and you probably need to do it again when jedit or bsh is updated