Someone was asking about this earlier today, and now that we're not busy getting XPack out the door, I finally have time to answer it.
We made some changes to the XPack code base to allow this.
And we added a flag to gauges.h: IMAGE_ERASE_ALWAYS.
Note that your gauge update rate will suffer a bit, since we erase the entire image before redrawing it again.
The gauge that we use for the 2D panel is 99% the same as the one we map into the 3D cockpit, but differs in that the 2D gauge has the new IMAGE_ERASE_ALWAYS flag applied and that its background bitmap is transparent (all black) instead of white.