Hey BC,
First: You did the right thing. You saved your passengers and your own life.
When flying A320, I was taught (I believe by the same people who trained your trainers, can that be?) that the FL391 restriction was due to the pressurization-certification. I know of a case where a captain on our fleet climbed to FL410 without any pressure problems.
A "colleage", who happened to be on the same frequency, telexed the company (instead of warning the before mentioned captain). The CPT was told by our fleet manager, that Airbus -due to the certification- just cannot guarantee enough pressurization at that level.
I doubt the oxy masks can cause the limitation. We use the same on our 340s, which are allowed to climb to FL410 (it only takes hours to get there... sigh).
One thing is for sure: There is no aerodynamical limit at FL391 for the 320-family, nor is the 320-family performance-limited at FL391. Flying in FL400 for a short while is no problem at all.
Shortly climbing to FL400 to adhere to your TCAS-SOP (=standard operating procedure!!!) is IMHO the absolute safest course of action.
Keep strong, keep safe!
P77