God forbid it ever happens but if there were a mid air between a glider and a powered aircraft in IMC I suspect the AAIB would have no alternative but recommend all gliders operating in IMC should be required to carry a transponder.
I just remembered having read something on this topic in the summary of the latest EASA proposals for FCL as published in Flyer Magazine, September 2008, pp. 32-33. Of course the article is a two-page summary of a 600+ page document so they might (will) have left something out, but I've got the article here in front of me and it says the following about glider licenses:
"An appropriate EASA Licence will be mandatory for piloting aircraft covered by EASA airworthiness regulations, often known as Annex I aircraft."
"EASA licences will be available for ballooning, gliding (sailplane) and airships, as well as fixed- and rotary-wing aircraft."
"There will be no IMC rating unless further regulations are proposed"
"There will be no 'cloud flying rating'"
Furthermore, if I read the summary correctly, the IR can only be attached to a PPL (or higher), not to an LPL or BLPL, and the IR would only apply to powered flying.
Now I don't know how much of the gliding community is actually going to be brought under EASAs authority (it seems to depend on whether your glider is an Annex I aircraft) but the whole business of gliders flying in cloud might be gone anyway in a few years.