These pretty much combine to tell me that since Empeg's libraries were directly ported to Java, they, and thus all of JEmplode including any other libraries I choose to link into it later, must be released under the GPL (or released in multiple "pieces", which isn't really practical for users).

Ah yes, looks like you're right. That's not how I'd thought the GPL works (I didn't realise that a translation would retain the original copyright -- I assumed the JEmplode authors would be considered the copyright holders). So JEmplode must be released under the GPL.

Peter