Chris, you could make a synthetic luminance using the Ha blended with L (and, indeed, the R, G and B if captured at 1x1 binning, weighted by exposure). And/or you can blend it with the R channel prior to loading up in the LRGB module. Do this by loading the Ha channel, then opening Layers, then opening R and doing a 50% blend (or whatever weight is appropriate).
A full tutorial by Ivo is here: viewtopic.php?f=7&t=236
and it's excellent.
I did an [Ha]LRGB for M20 recently using StarTools: http://www.astrobin.com/134243/