Originally Posted by
DanK
with respect to 120 cd/m^2: this will affect the difference in brightness but should have no bearing on the handling of out of gamut colors. Calibration of the colors of the monitor, on the other hand, might.
What paper are you using?
Desaturation will indeed move the color toward gray. It may help recover detail when an area is oversaturated, but it doesn't change the hue, so it won't bring the color back within gamut.
Once your calibration is done, you can fiddle with the out of gamut color in Lightroom, for example, by moving the relevant hue slider(s) in the HSL color panel in the develop module.
IMHO, this ultimately comes down to making changes that you consider acceptable. If a color is out of gamut, you have to change it one way or the other, or let the print pipeline change it for you. The ultimate test is whether you find the result acceptable.