Well at least you're in famous company - Cream, Albert King, Jimi Hendrix. I'll commiserate but short on advice. Sounds like you're doing all that can be done. I once had an bike oil leak that only manifested when the motor was off (& it wasn't that bad). While she was running, not even a drip would appear. Sounds like that might be your situation too. I know you checked for the presence of the oil pan drain plug. Good luck for a change.