First post here so let's make it a good one. I bumped into the Map\ folder idea for SD cards by reading a thread on poi-factory.com.
I tested it on my 1350T running the latest firmware (SW Ver 5.00), and it works:
You can store multiple img files in the X:\Map folder, and you can name them whatever you want. By default there is no Map\ folder so you'll have to create one.
I have files named Europe-2011-40.img and TOPO-BeNeLux.img in that folder, and it's being picked up just fine. In fact, I don't have an img in the Garmin\ folder anymore right now.
Have fun and thanks for the site. It's been a great resource so far!