I don't think anyone would risk getting banned, given the Strix platform is run by the administration of this forum (or at least one), if memory serves me correctly.
UPDATE: Good point made below. I might have misunderstood the question.
For all intents and purposes, except for heavily obfuscated builds, you don't even need the sources of an L2J build to make changes to the core. You can extract, deobfuscate, fix some loops logic, and recompile or inject back into the jar.