Not fretting over toluene - just noticed in a study that they mentioned animals were given high doses of isos and cancer was the result. I went and reread some parts of other studies. When you don't know anything about chemistry, then you're guessing (like me).
I follow-up read suggests the following - toluene diisocyanate was listed as carcinogenic in a study (I think it was fed to animals), but no carcinogenic link found for inhalation.
That's not exactly reassuring, nonetheless.
Reading about isos in general (and potential misinformation on guitar forums from some folks suggesting that the isos will persist in a vented garage for weeks and contaminate a house and everyone in it, for weeks) probably isn't instructive to a level I'd like to see, so all I can conclude from all of this is that the best idea would be to just look elsewhere. It worked fabulously on a car hood, but I don't need the same guessing just for the desire to put a hard high-gloss resistant (permanent non-repairable) finish on a guitar.
I sprayed at the edge of my garage with a respirator on and the garage door open, and the door between house and garage blocked (towel under gap, etc) after reading warnings about it back then, and then went back into the house through the front door so as not to open the basement door. I thought I was maybe being a little over the top at the time (now that I recall), but probably not.
The aromatic nature of nitro lacquer leaves it out for me, too - I'll just look elsewhere, which is kind of a bummer. The toughness of the finish and the convenience of a trick two-part can (you puncture an inner cylinder in the can when you're going to use it, and then it obviously has to be sprayed within a short period of time or it'll just harden in the can) so that you don't have to clean spray equipment was nice.
I also see now why some of the two part foams were always warned against as not being appropriate for "playing around". Due to isocyanates.
The iso-free 2k poly is available and looks like it has been for about a decade with a comment that it can lose lustre over time (everything probably does) faster than the iso-containing prior versions, but I'll keep my eyes peeled for an offering of the iso-free stuff in cans. It looks like the spraymax-ish version of the 2 part clear in a can is sold under a bunch of different labels (eastwood, spraymax, etc) but is the same thing everywhere.