Jim, have you experimented any with flight following? I've not used
it, but allegedly, ATC with warn you of other transponder equipped
aircraft. It's not the same as carrying a warning device in the
glider, but it might have some utility, and it's cheap!

ATC doesn't generally LIKE it, but they've given me
flight following before with no transponder. I let them know
who I am by doing a 360 (easier than trying to turn
to a compass heading THEY call out).