I think it'll work fine. I have a Swift that I think is just a bit too narrow for me even with a sorta-aggressive road bike position. But, I've surely got wider sit bones than you anyway.

My concerns about using a Brooks MTB'ing would be catching your shorts on the edge of the leather when getting on and off the saddle and, maybe, having it dig into your shoulder if you carry the bike with the saddle on your shoulder.