Er... I think you missed the point ShyTorque...
99% of all gliders are fitted with radios; those radios, almost without exception, operate across the full Air frequency bands. Those that aren't, are normally old vintage aircraft which are very unlikely to fly x-country.
Most tranceivers are designed to draw as little current as possible when on TX/RX thereby providing maximum endurance for the battery or batteries. (Typically up to 8 hours)
The issue is regarding fitting a transponder not a tranceiver. Transponders consume significantly more power than a typical glider tranceiver, and the additional electrical load would significantly reduce overall endurance, without the need to add additional batteries (which add weight/Cof G changes etc). Most gliders are fitted with a single 12V7AH battery; some have 14V 7AH; some have a reserve.
When you consider that it is possible to fly distances of 500kms cross-country, with durations over 8 hours, then the overall electrical load needs to be as low as practicable to ensure sufficiant power is available at the end of a flight.
Once you start adding GPS systems, horizons (which can draw 1.5A on start and 1A in a steady state), Mode C transponders etc then electrical endurance for these long flights (where you may be more reliant on additional instruments) does becomes a issue.
You also stated that:
However, if a glider pilot wants to venture further afield, I think for his own safety (and everyone else's) he needs to consider the bigger picture before he goes flying in cloud without advertising his presence to others
As both myself and other posters have already pointed out; cloud flying is the exception not the norm. It's not done every time you fly ouside of gliding range of an airfield as you would seem to intimate. And as already mentioned in this thread, it is standard practice to call before, during and leaving cloud, when you do enter it using the en-route frequency and cloud flying frequencies.
The majority of the time we fly in Open FIR, not under control of an ATC - I'm not sure that there are too many other frequency options available?
Glider pilots just don't fly cross country willy-nilly and I can assure you we are more than aware of the bigger picture.
Before a glider pilot can even begin to fly x-country he or she requires compentancy and endorsement (At least 2 successful approaches into fields; planned and flown a triangular task of at least 100km (in a glider, motorglider or light aircraft) and having already passed examinations on airmanship, meteorology, principles of flight, radiotelephony and navigation).