I didn't see any solution for using a Tivo as a Mythtv tuner; if there's already one, please clue me in.

This proof of concept waits for the show to finish recording and then transfers it to the mythtv machine for playback. I'm interested in making it live stream the recording, but I haven't seen any way to do that yet - still reading the mfs_ftp thread, etc

This uses the mythtv Import tuner and the events interface (requires mythtv 0.23 iirc). In mythtv-setup, add an Import tuner (or two) and connect it to an appropriate "Video Source." Then, in the System Event editor, add a comment for "Recording started" like: /usr/local/bin/mythtv-record-event "%CARDID%" "%CHANID%" "%FILE%" "%STARTTIME%" "%ENDTIME%" "%TITLE%" "%SUBTITLE%" "%DESCRIPTION%" then install the attached script.

It requires a fix to mfs_ftp.tcl fsids are not updated before a RETR, so one must do an ls before a get, but the script already knows the name.

Code:
 proc RETR { fsock line } {
 global info db control_socket resume ; set p 2
  set line [string range $line 5 end]
  outd 1 "RETR:\n\"$line\""
+ update_rec_fsids 1
  set line [file tail $line] ;
I haven't tested with two tuners yet, but it should theoretically work.

mythtv-record-event.zip