Now this is a little silly, but are there any manufacturers that
make modern Mode "A" transponders (no altitude encoding)?
I've seen a portable one (battery operated) that looked pretty
old.

I think this is TSO-C74b, ref 91.215(a). I just wonder how much
of the extra price and inspection is just the mode "C" part of modern
transponders.

I wonder how many more owners would install a mode "A" if it only
drew 50mA max, was the size of a cell phone, required no recurring
inspection, and cost "about $100" (quote from Rain Man).

Better than nothing. And allows you into A,B,C by deviation
request. And gives at least lateral avoidance to airliners.

Maybe a market here to compete with the over-capability of the
mode "C". Better is the enemy of good, especially when talking
about optional equipment.