I've just tested the theory; loading OSM mapping for Australia by writing from the PC installed map-set using MapInstall to an SD card. This created the correct Garmin folder on the card and wrote a GMAPSUPP.IMG file.
I then plotted a route in Basecamp from just outside the Sydney Opera House to the race circuit at Phillip Island and then exported this to the same SD as the OSM based GMAPSUPP.IMG file created above. Route is now running in simulation mode on my Nuvi 765, all seems good
I then took this one step further combining the Aus OSM GMAPSUPP.IMG file with OSM mapping for Florida creating a new GMAPSUPP.IMG file using
GMapTool. I wanted to see if I could combine OSM map-sets into one as there appears to be a limit on how much data you can include in one download and you can apparently only have 2 or 3 of GMAPSUPP.IMG files on a Garmin GPS (later models may allow more). If you wanted/needed to create a GMAPSUPP.IMG file for the whole of Europe for example you could do this with GMapTool, combining the readily available map-sets for each country. In my tests so far this works well and runs the above route without issue. It's a 600 odd mile route so I'm probably not running it over the full distance.
Of course with a 2610 you would run into the available space issue before completing a map-set of Europe but you would be able to combine countries or just the required tiles together into a single map-set for a given tour up to the 2GB CF card limit.
However, I then remembered, from my days owning Quest, that you could in fact do the same thing with Mapsource, selecting tiles from one map-set, swapping the map-set and then selecting further tiles from that before 'sending to device'. Well, it appears that you can do the same thing with MapInstall as well. Select the tiles you require and then swap the map-set and continue to to select tiles from that one combining parts, of all, of two or more map-sets into one GMAPSUPP.IMG file.