For what it’s worth… I was running Processing headless on a Raspberry Pi and had a similar question. It turned out to be easier to just use --export, but to answer your question:
Edit the processing-java script to enable logging and look at the class path it builds up, which contains all of the Processing JAR files. To this you also need to append the path of your sketch’s .class file, the same path you passed as the --output directory.
However if you try to call save() or similar, you’ll want to use an absolute path. I tried setting user.dir which is the property from which the sketch path is derived but it did not work.