So the Tivo somehow managed to fix itself overnight last night. Further research indicates that it sometimes take it that long to implement those kinds of settings changes, which is pretty sizable bag full of suck, IMO. Why does it take as much as 24 hours for a simple settings change to take hold? Mind you, it was set to accept transfers from the factory but didn't work, so I had to turn transfers off, turn them back on, and wait overnight before I could use that functionality to begin with. Bleh.
So now transfers are working both ways. Hooray!
I found a great Mac program called VisualHub that can transcode just about any video format to a Tivo-compatible MPEG-2 format, and also plugs in to the Tivo Desktop software on the Mac and allows it to transfer video back to the Tivo
Going the other way, it turns out the Tivo has a webserver built in that you can log onto to download videos off the Tivo on any platform. Using the URL https://yourtivosipaddress/nowplaying/index.html from within your home network, and then using the user name “tivo” and your Media Access Key as the password, you can get at any of the video stored on your Tivo!
But… and it's a big but… the video is still in the encrypted .tivo format, so you still have to crack the DRM before you can do anything with it. Sadly, there are no Mac programs for this so that remains the one step in the process that I'll have to switch over to my PC for. Still, it's more than I had planned on being able to do on the Mac, so it's all good!