You cannot 'tell' ColourSpace to measure a smaller gamut, unless you use Sub-Space.
ColourSpace will always measure the full available gamut of the display.
(As per the User Guides, the Target colour space setting is irrelevant during profiling - it just gives the graphs something to plot against.)

So either there is some colour management active in the signal patch somewhere, or that is just the max native gamut of the projector.

Steve

Yep, it was in the signal path. To make sure, I did a full factory reset of the Lumagen, and.....now I am seeing the full gamut looking similar to how I expected it. (I'm measuring of the screen with an i1D3 -- ColorMunki to be precise -- hence the terribly high luminance reading of a little more than 1000 nits.)

Cant edit that post. It should read that I was NOT measuring off the screen, but off the lense, thinking that might be more accurate / neutral for assessing the projector.

Now I am reading off the screen since the i1D3 has no trouble reading the black level of this Jvc projector. After putting the projector into bypass mode, and adjusting the white point for d65, profiling the behavior is looking a little weird.

Is there a likely cause for this? Using Lumagen as the tpg.

Yes - something is very wrong with the profile, as can be seen in the RGB Balance and EOTF graphs.
Something in the signal path, but could be anything.
Could even be incorrect Extra Delay setting?
But impossible to say.
You will have to test different things to isolate the issue.

Steve