Having never really dealt with gliders transiting class D, what kind of level would you usually be requesting? And would you be able to hold if required? That would be the 2 main questions I would be asking as a radar controller. The answer to these questions would determine if IFR traffic were going to be delayed and subsequently if your transit will be approved.
Gliders using thermals to fly cross-country tend to use the height band from cloudbase down to halfway to the ground so if cloudbase is 4,000' then from 2,000' to 4,000' is the preferred soaring band. Of course gliders cannot maintain a specific level - they are always descending or climbing. If you get lower than the halfway height then you start to move from cross-country mode to 'survival' mode; trying to stay up becomes more important than trying to make progress on track until a better height is regained. When crossing a MATZ it is common to be asked to report if you are expecting to descend below a minimum height.
Holding is a bit tricky in that circling in sink will result in the glider being on the ground pretty quickly! However, a glider can circle in lift for quite a time - although it will be drifting with the wind. Depending on conditions it might well be feasible to fly to a nearby thermal to wait for onward clearance.
A modern glider should be able to achieve 6-7 nm for the loss of 1,000' (through the airmasss) so getting high before e.g. crossing a centre-line should result in uninterupted progress. Assuming a couple of thousand feet is available for a glide through a zone an uninterrupted 10+ nm glide should be achievable. If flying straight up or downwind, following lines of lift, often much better is achievable - sometimes 20-30 nm between thermals.
Gliders will usually have an airspeed between thermals in the 70-90 kt range.