That's a good point about the head tube length. I have a suspicion that is the measurement I should look more closely at. I don't know why I didn't consider it before. I'm now putting together a little list of large, disc compatible bike frames with the measurements for comparison.

Also, in the course of looking more closely at measurements, I noticed the measurements of stack and reach, which I think is probably exactly what I want. Besides rec's for large bike frames, anyone with any ideas how to calculate those measurements from online measurements?