I'm interested in this discussions as well. I have rims/hubs/spokes in 650b size waiting on my bench to be built so I can experiment on my two favorite road bikes. Brake reach is less of an issue with my target conversion bikes because 25c is a tight fit on both. I'm finding tire width to be one of my limitations --with 650bx32 looking like a tight fit. I'm also considering bending a higher rake fork to go even lower on trail --just to experiment. And feel in loose gravel is one of my target experiments. I have felt like the steering on my 57mm-trail (other) roadie (even with 32c tires) wants to plow a trough while I'm trying to wrestle it back into balance. I had thought big tires would solve my gravel issues but it didn't seem to make much difference. I want to see what low trail feels like in gravel. (I'm also building a loaded tourer from a 1988 Trek 850 with 62mm trail and am anxious about its ability to handle gravel)

I don't have answers yet but I'm pursuing similar experiments and will post what I discover.