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.
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