Wouldn't it be cheaper/simpler to run it as a 3sp considering you've already got the BWR hub/wheel? That's how I've got mine setup and it's been grand.
Most modern frames should have shorter seat tubes to enable you to run longer droppers. It's not unheard of being able to run a 200mm dropper on a size large @ ~6ft given the frame is appropriate of course.
It's the Townsend family's long-standing shop in Cambridge. Lloyd Townsend is current owner (4th gen) and also Ison MD