Nice idea. Actually it can be done even simpler.

Prepare directory for maps, just like you have described, name and drive are example only:
mkdir "E:\Garmin maps"

Create a subfolder "Maps"
mkdir "E:\Garmin maps\Maps"

Right click on Mapsource or BaseCamp icon or position in start menu and then choose "Properties"

[Only registered and activated users can see links. ]


In dialog window fill "Start in" field with name of your new directory:

[Only registered and activated users can see links. ]


Now copy all gmap folders you'd like to have on this drive into "E:\Garmin maps\Maps" directory.
For unlocking maps simply use Mapsource or BaseCamp menu:
Utilities/Tools -> Manage Map Products -> Unlock Regions -> Add.