Mode S transponders

Anyone know if there's a lightweight mode S xponder in the works that's
practical for cub types, gliders, whatever? I live in Europe, and the day
is fast appraoaching when these will be required on all aircraft flying in
or under controlled airspace, effectively confining all antiques to their
own field. I was wondering if a lightweight, battery powered option that
doesn't cost half as much as the airplane might be under development
somewhere. I've searched the net in vain, including google groups.

Anyone know?