Is it possible to play a high C natural without a key(or with one)? 
Personally I can’t but it’s been suggested that you can:
http://chiffboard.mati.ca/viewtopic.php?t=28245&postdays=0&postorder=asc&start=0
Scroll down to Uilliam’s post, about 2/3 down the page.
If you have a very good relationship between your chanter and reed it is possible… but very rarely reliable.
PD.
Paddy Keennan said that it was doable, but very hard to hit.
I can’t recall him actually showing us how to do it at the Tionol, though.