While we wait for LT to add this feature I've been playing with multiple btrfs pools with the help of UD plugin for some time with, while there are some limitations it's been working great, I wrote a FAQ entry for anyone who wants to try it, but note it's work in progress, it should be safe but use at own risk:
https://lime-technology.com/forums/topic/46802-faq-for-unraid-v6/?do=findComment&comment=462135