JA,
What you're suggesting is already being done.....
A spectrograph - which disperses the visible wavelengths allowing each wavelength to be recorded is used. All the, say, emission wavelengths of the target, Hb, OIII, Ha SII etc. etc. are independently recorded and can be recombined as required.
http://www.astrosurf.com/buil/scan/demo.htm
(use google translate if your French is poor ;-) )