Well, by combining information from a couple different threads here, I have it working right now. In fact, I'm watching a show right now on my Tivo HD that I transferred back from my iMac. Here's what I did:
1) Enable the hidden "Video" transfer feature in Tivo Desktop 1.9.3 if you haven't already done so. To do so, turn off Tivo Desktop in System Preferences, then paste the following into Terminal:
defaults write com.tivo.desktop FileVideo -dict-add VideoUIEnabled -bool true
Restart Tivo Desktop, and you'll now have a Videos Tab. And on your Tivo under Now Playing you should see your computer.
2) Download the Tivo Decode Applescript at http://inik.net/system/files/TiVo_Decoder.zip
3) Presumably you already have some Tivo shows on your computer in the folder user/TivoShows. Simply drag one of them onto the AppleScript you downloaded above. I found that a 30 minute show converted in around 30 seconds. This AppleScript strips the Tivo wrapper and leaves you with a basic MPEG-2 movie.
4) Now go to your Tivo. The show you just converted should appear under your Computer in Now Playing. If not, try turning Tivo Desktop off and on again. Transfer as you would any show, and enjoy!
I've noticed a couple issues, though they seem minor:
One is that on your Tivo, the Fast Forward/Rewind bar may show the program's duration in seconds, not hours/minutes.
Two, for some reason, if I tried to play the converted shows on my Mac with Quicktime Player, they would play with audio, but no video. And I have the Apple MPEG-2 plugin installed, so not sure what's up with that. FWIW though, the converted show played fine in Toast Video Player.
Third, this does not appear to be a workaround to transfer an HD show to a Series 2. Even converted to generic MPEG-2, the Series 2 does not seem to be able to play a transferred formerly HD show.
I'm running the above setup with a Tivo HD running 9.2 and a Series 2 running 9.1.