I tried adding a User Airspace as Class G, but LNM displayed it as Class E. This was the file I used:
AC G
AN EGTB ATZ
AH 2000 AGL
AL SFC
V X=51:36:39 N 0:48:28 W
DC 2
If I'm reading the code right, I think it's airspacereaderopenair.cpp that's changing it from G to E (and F to E as well)?