In the early 70s, when the series was on TV, I was in Leipzig attending the Trade Fair. One weekend, with nothing better to do, I decided to visit Colditz. Of course, they wouldn't let me in because, as the very helpful lady at the door explained, it was currently a lunatic asylum. I asked about the glider. She knew of it, and when asked if it was still there, answered
"Ich glaube so". (I believe so). For some reason she became convinced that my father had been incarcerated there, (he hadn't), and she suggested that I went down to the pub in the nearby village square and talked to the locals. The locals were very friendly, and a couple of them worked in the castle, but they said that a lot of it had been closed off and only the maintenance people were allowed in there, so they weren't sure whether the glider was there or not. None of which answers your question very well, but it might be worth firing off an inquisitive email to these people:
Information - Schloss Colditz
It was a dull day, cloudy and raining when I was there and as I stood on the platform at Colditz station waiting for my train back to Leipzig and looking up at the building, I found it very eerie and forbidding. It was still in the same condition since being captured by Allied Forces. Of course it is now a museum, fully refurbished and open to the public.