If ATC allows it then it’s fine, you can try? in UK, one likely to need IFR flight plan & IFR rated to operate VFR above FL200 (tried once with a friend and it was ok to go up & down few times when testing with and without oxygen), UK gliders & military can fly VFR up to FL240…
Do Gliders have some arrangements on this? I am sure I saw +20kft records in the clubhouse in Bavaria
There are special wave flying zones which are activated on demand. And just recently I watched youtube video where Klaus Ohlmann said, that it’s always worth to ask ATC, even if it doesn’t always work. He does the crazy stuff:
https://www.flieger.news/von-serres-nach-thessaloniki/
Just look at the altitude profile, right before the leg between italy and greece he’s at 7000m
Sure if ATC allows it, you are good to do anything (except VFR in Class A which I have had clearances for in Spain and Italy 
). The gotcha is that they can allow something and then after a few mins get a bollocking from a higher authority, like happened to me near Norwich when I was doing an altitude test flight… I got to FL210 and it was stll climbing nicely at 300fpm (it was -39C!) and London Control gave Norwich ATC a bollocking 
Peter wrote:
Sure if ATC allows it, you are good to do anything (except VFR in Class A which I have had clearances for in Spain and Italy ). The gotcha is that they can allow something and then after a few mins get a bollocking from a higher authority, like happened to me near Norwich when I was doing an altitude test flight… I got to FL210 and it was stll climbing nicely at 300fpm (it was -39C!) and London Control gave Norwich ATC a bollocking
What stops you from officially flying IFR above FL200? Your celing is 20000ft, not FL200.
