Doesn’t plan9 support frame buffers over 9p or something like that? You could probably write a wrapper that just forwards a Linux browser to a plan9 window
Many years ago a roommate and I had an HPUX machine running IE on HPUX just so we could forward X session to our FreeBSD and Linux desktops and not have to use our Windows machine for anything other than PC games.
It's probably easier to just run a VM directly into a window.
There are solutions, like VNC to some UNIX-ish machine, but, yeah, a native browser would be cool! 9front has a hypervisor, you could run something in there. https://man.9front.org/1/vmx