Steps to build a custom Hibiscus plugin for Jameica:

• Checkout the code: The “hibiscus” module from the Hibiscus project, the “jameica” module from the Jameica project: “cvs -z3 -d:pserver:anonymous@cvs.berlios.de:/cvsroot/hibiscus co hibiscus && cvs -z3 -d:pserver:anonymous@cvs.berlios.de:/cvsroot/jameica co jameica”.
• Within jameica/build run “ant nightly”, which will generate releases/jameica-lib.jar
• Edit hibiscus/build/build.properties, make define.jameica.dir point to the Jameica checkout, relative to the Hibiscus root directory (not the build/ subdirectory).
• Running “ant nightly” within hibiscus/build gives you a hibiscus-XXX-nightly.zip that can be installed into Jameica.

Update 2013-12-01: Still works the same way using the git versions.