I'm suggesting that Shellvault could operate an untrusted intermediary by terminating SSH in the browser instead of on their server.
If you don't believe it's possible, load this: https://bellard.org/jslinux/vm.html?url=https://bellard.org/... and use the SSH client. It works already.
You do have to trust Fabrice Bellard not to ship a compromised client to the browser, but a.) that's much less trust than you'd be giving to Shellvault, and b.) there's no reason you couldn't self-host the files for jslinux.