wonder if a quick email to them could get them to add that feature? seems like a side project for them all, but sounds like an easy enough (and obvious) feature.
i did a quick look for multi-tenant ruby saas boilerplate, and saw the usual list of things that don't quite do that, but all of them seem to do at least single-tenant -- and prob most would claim they do multi-tenant if you just add 5 or 50 hours of dev:
https://www.getsjabloon.com/
i don't know anything about these particular guys.
but my thinking is, you can get to (what seems like) 80 or 90% in a few hours, but that final 20 or 10% takes 50 hours more, plus ongoing maintenance/support/etc.
i'd pledge $5 at six months from now (possibly more) to see it done - just to try to kickstart it.