Is it legally possible in Europe to get clearances for IFR, night VFR or special VFR while flying gliders? Specifically, I'm interested in sailplanes, powered sailplanes and ultralight sailplanes (MTOW below 472.5kg).
It's not possible to get IR for any of those. I think EASA now has flying in clouds rating for gliders. For night flying you will need a glider that is certified to do that. I don't know any manufacturers of night gliders. Blaniks had an option to fly at night but most of them cannot fly now at all.
SVFR may be possible in theory but it will be crazy. Maybe you can be towed in special VFR but this will be dangerous. It also depends on whether the country in which you are flying allows SVFR.