Is it legal/possible/reasonable to install just Mode A transponder (No
altitude reporting)?

Whenever I hear "Traffic 12 O-clock 3 miles, altitude unknown" I look
EVERYWHERE. ahead. Might this provoke more visual scanning by aircraft
crews which could spot gliders without transponders in the same gaggle? A
case of one protecting all?

As a side benefit this would avoid the 24 months Mode C altimeter/encoder
check.

Bill Daniels