Had another go tonight. For sake of realism, I kept all processing operations global (applied to entire image). The two more unusual things I've used:
1) GIMP's convolution matrix to manually shift the red component up by 1.2 pixels,
2) the "Dilate" filter to highlight the structure of the nebula. The joining of the two close stars in the nebula is an undesired artefact of this filter.
Rest is just several level tweaks and Gaussian blurs.
http://members.optusnet.com.au/~janosk/misc/5189-1.png