Ooh, and you can "self host" this thing, at least as much as you can "self host" a serverless *thing* that runs a VM entirely on the client side.
https://github.com/leaningtech/webvm
I've forked the repo and now have my own webvm running on a custom domain. Next step: mucking about with the Dockerfile to tailor it to my own needs.
This is gonna be fun!