I know very little about the practical implementation of TOS and new tricks are not my forte these days!
However, I would guess that it is not as simple as this.
The issue, in my mind, is not shrinking btrfs, but in moving it's start point and telling the system where that start point is. If it only concerned one disk it might be doable, but with several disks in a raid formation .....
My experience of "old" raid says it won't be pretty

Edit: Looks like we were cross posting.