Sweeet... I copied zipfldr.dll to jarfldr.dll, opened it in 'Edit' (edit.com / DOS Edit), did a search and replace for ZIP with JAR (case insensitive match), and registered the DLL. It works, I can now search Mozilla chome jar files 🙂
edit: wow, that didn't even make a mess of my registry. Now to find the "right" way to do it.