Lossy output with FFMPEG JPEG 2000
Did you try using the -compression_level flag? It takes an int from 0-100 afaik (maybe the range is even higher, the docs just say "int").

That should set a lossy compression strength.

There is also an interesting blog post about creating DCI compilant JPEG2000 encodes with FFMPEG for DCP's. Which require lossy encoded JPEG2000.