No help for Mark i'm afraid, but I'm facing a related "issue" working on L-Ha-RGB data for M1.
Outside the core (synchrotron) region, Ha provides most of the flux (i guess a smiliar issue applies to the faint H-alpha outflow in M82).
If I add the Ha to RGB, then merge the HaRGB with L, I lose the outer filaments, because there's little "L" signal there.
If I add the Ha to the L, then merge with RGB, I lose the colour in the outer filaments (because there's little RGB there).
Any suggestions welcome!