Recover from a broken plan9.ini

At the boot prompt, enter:

!rc

This should drop you to a minimal rc shell and environment.

To mount 9fat:

; ls /dev/*/9fat
/dev/sdF0/9fat
; dossrv -f /dev/sdF0/9fat 9fat
dossrv: serving #s/9fat
; mount /srv/9fat /n/9fat
; cd /n/9fat
; ls
…
plan9.ini
; 

You won't have a full environment, but sed, grep, mv, and awk will be at your disposal.

Good luck.

Personally, I keep a plan9.ini.old file every time I make risky changes.