Couple of thoughts (ex-UK glider pilot and power pilot):
On Transponders - Are we presupposing that all the outside CAS IFR occurs in areas that are subject to radar surveilance, thus making the transponder useful?
That may be a daft question, but in my locale, there's vast tracts of class G that aren't covered by radar. Given that only a small percentage of GA IFR aircraft have any form of TCAS, transponders would seem to add very little in that situation in most cases. (appologies if that's irrelevant to the situation being discussed).
What we do out here however, is have area frequencies, on which everyone (gliders included) should be listening, and generally make broadcasts of our intentions - such as tracking from x to y at z thousand feet, eta y such and such a time. Not perfect, but does aid situational awareness.
On gliders in IMC - Don't forget that technically the glider is in IMC (or at least not in VMC), when it is close to the clouds - if you limit gliders to VMC without a slew of kit, and whatever is required to power it, you also limit them (technically) to remaining 1000ft and 1500m clear of cloud etc. Now I'm not sure that many glider pilots cloud fly that often (I for one would not care to for many reasons, including not wanting to finish up wearing a light twin or similar), however having to maintain VMC cloud separations (particularly 1000ft under) would present a major headache. Especially with typical UK cloudbases; perhaps a reason to defend IMC rights so vigorously.