I've successfully installed 2011.10 to my PC (using the full Mapsource version), unlocked it, and transferred a small part of the map to my 265WT. It was small enough to fit in the remaining space on the Garmin itself. And it looks like it works! :)
A couple of more questions:
1- Let's say I want to remove the GMAPSUPP.IMG file (the small mapset created) on the Garmin and replace it with something else. What's the process? Do I simply delete the IMG using Windows Explorer, or is there a way to do it within Mapsource?
OK, I figured this one out - looks like only one supplemental map is allowed and the first is over-written