I think this is neither art nor photography but instead data visualisation, and in fact a valid thing to do. There are lots of examples in science and medical imaging where a grey scale image with a high dynamic range is visualised with pseudo colour mapping.
As a quick and dirty example on a stretched mono image (eg Ha) you could try
R: abs(1.0 - $T)
G: abs(0.5 - $T)
B: abs(0 - $T)
[EDIT] Attached is a Lagoon Neb Ha image. The colours really aren't the best for showing the detail, but as I said, it was quick and dirty!
With trial and error you could adjust the constant values in each expression down until it could be applied to an unstretched image. There are lots of other (better) pseudo colour palettes that could be used but they would use more complex RGB expressions.