... If there's a way to test a particular defined color as to whether it's visible or not.
The information you seek could be here: http://docs-hoffmann.de/cielab03022003.pdf
Scroll down and you will see colored squares in L*a*b* 2D form, marked whether they are in gamut or not, for the popular color spaces. There are many pages devoted to different values of L*. Most pages show the human vision gamut in the background, 2D though.
Bruce Lindbloom's site has a wealth of information:http://www.brucelindbloom.com/UPLab.html
There, you may conclude that hues aren't too linear in L*a*b* and that gamut assessment is perhaps secondary to perceptual color comparison (delta-E).
Should keep you occupied for a minute of two
I looked at Adobe Wide Gamut in ColorThink and it seems quite close to human vision (CIE 1931, 2 deg observer) except in the cyan zone.