As to accuracy of the transponder, when the QNH is close to 1013 I can read the flight level against altimeter; I believe the 25' accuracy statement!
My usual aerobatics height band is between 3000' altitude on the local QNH, and FL55. I leave the altimeter on QNH, and read the FL from the transponder. Works fine. You just have to keep in mind that if the QNH is < 1013, the height band available might be (significantly) less than 2500'.
My only suggestion is to ask if you kit-out with a radio and talk to the LARS service?
A lot of gliders have a radio but do not have their FRTOL. So they're only allowed to operate on specific glider frequencies. That rules out getting a LARS.