With a few external tools you can set this,
http://www.gmaptool.eu/en/content/map-visible-basecamp
or http://www.javawa.nl/jdm_en.html

Don't know if such option could be implemented in mkgmap?
Hi developers,
is there an option to make the gmapsupp visible/invisible in Basecamp?
At the moment its always visible, but this is a bit annoying (since one has to wait some minutes until some hundred MBs are copied).
Thanks for your ideas,
Joern