We are aware of this request and it's on our famous things-that-we-would-like-to-do-someday list. There are no concrete plans to implement this in 4.3 though.
We are mainly concerned that there is no way to exchange avoidances between devices and BaseCamp, which will lead to different routes created by BaseCamp and the device. It looks like more devices are adding support for taking routes straight from BaseCamp (without recalc), so adding avoidances looks more useful as well.
