I made something work, and it was difficult enough that I thought I would share it with others.
I have a Netgear WNDR3700v1 wireless router with a USB port. I connected up an external HDD to it, with the plans of sharing music, movies, and photos to my S3 HD Tivo. I worked on this problem for a past month or two, and just recently got it working. This functionality is completely independent of Tivo Desktop, or Tivo Desktop Plus, and doesn't require Tivo Desktop or Tivo Desktop Plus to be installed or running.
With the upgrade of my Netgear WNDR3700v1 wireless router to Firmware 22.214.171.124, I now have all of my music, movies, and photos content available via my S3 HD Tivo menu.
If you need to troubleshoot, at the root of your attached USB external HDD, change your Windows folder options to display hidden files. It is then you will see a .ReadyDLNA folder, and under that will be additional files and folders. If you are having trouble displaying the content on your Tivo from the attached USB external HDD, check the "minidlna.log" file for content indexing information and errors. This helped me determine if some of the music, movies, and photo files on the external HDD were corrupted or otherwise not able to be scanned and cataloged.
Remember that when you add additional content to the USB external HDD to make available to the Tivo, you may have to go into the device management screens of your Netgear router and toggle the enable/disable Media Server feature of ReadyDLNA, or just toggle the rescan of media option.
Hope this helps. Post here if you have questions.