Thanks to everyone for the feedback about transponder gauges. I certainly have learned a lot about Flight Simulator gauges in the last few days! After further investigation I have decided to implement the in-simulator set of transponder controls as a FS module based window. Writing it as an actual gauge will make installation unnecessarily complex because gauges have to be installed into FS on a per aircraft basis.

Here's a mockup of what controls the new window would have. Notice my impeccable skills with the graphic arts.

In addition to this, SB3 will also put the necessary information into some custom FSUIPC offsets so that gauge developers can make SB3 transponder controls available on their own custom gauges. Watch this space for details about this API.