Rod, with the amount of time you have spent on here in the past telling everybody how easily you can do pilot maintenance on your permit machines, why not re-make some parts out of lighter materials, shave a few kg off the empty weight, and then you could fit a transponder.
I don't think the
powered planes that truly have no electric power available for accessories represent a significant hazard to other traffic. They tend to fly at very low levels.
Your requirement for a mobile phone sized unit with an integral battery is a bit silly. Why not have it fit into a wristwatch - would be even smaller then
