The one I saw took off from a trolley that ran on rails, alongside the regular runway and landed on skids.
This site has a write up on the Leduc program and some of the photo's are at Istres :
Leduc, the test flights
There was also one of the Languedoc 'launch' a/c parked at Istres all of my stay, I never saw it used.
As you say, it was a terrifying m/c. As for security, in my time we never got anywhere close to it and the RAF Liaison Party was moved to Orange in May 58. There was some talk about the move being a 'security' thing.
At Orange the French had one squadron of Mystre IVb's. 2/5 escadron de chasse.
As a matter of interest, I still have my Laissez-Passer for Orange! But I think I look a little different to the 52 year old photo, so they probably wouldn't let me in today
