God we are sad b**tards. If you're in a telnet session - or I suppose using serial (don't know never tried it) - (ie you can run "nano" and edit a file), then all you have to do is type the command "rw" at the tivo command prompt BEFORE editing the file (thus making it writable), and then "ro" AFTER you're done saving it so as to set the partition back to "read-only".
Go to bed NOW, google for Steve Jenkins' network hack howto tomorrow and the mount thing will make more sense.
Clive