I am happy to announce that I have now managed to get JSS and the rest of ABCL-CONTRIB [working under Java 11][1].
[1]: https://github.com/easye/abcl/tree/jdk11
In the end, the total amount of changes needed was actually quite small. The current changes almost entirely stem from move in Java11 to use the new module system for all system artifacts in that they are no longer distributed in jar archives, but instead use a new URI scheme of ‘jrt’ (presumably an abbreviation for “Java runtime”).
Combined with anecdotal evidence that the adoption of Java11 is proceeding quite a bit slower than ORCL had perhaps hoped, I no longer think that forking the ABCL codebase into two versions supporting pre and post Java11 makes that much sense at this time. Instead, I think I will clean up the current patch, commit to the main repository, and prepare an abcl-1.6.0 release that will support Java 6, 7, 8, and 11, skipping Java 9 and 10, which are EOLd at this point anyways.
Comments and experience with the patches are welcome as always.