Hmmm... serf's parma... can you summon a magus in final twilight? If not, that would leave most of the founders out of reach. The background seems to imply that this is the case, but I might be wrong here.
Xavi
No, but it has been suggested that they might exist in the magic realm (serf's parma on the second part, noble's on the first)
That was where I was going. IF final twilight forces a magus into the Magic or Faerie Realms, then a story (several, more likely) could revolve around finding them there. If not, another might be experimenting on Enigma's Gift rituals to find a way to transition to Twilight deliberately and somehow maitain consciousness and memory in the process. How warped do you want your Criamon to be?
Hahaha 
I just wanted to add though, if he did retire to the monestary, then he would have died a proper mortal death and would be buried in a crypt somewhere.
Then again, maybe he is still alive posing as an elderly monk.
Who knows? With a figure of such mythic proportions, anything is possible. Story seeds are endless.
But there is no denying Flambeau was a true man, and when (if) he died he died like a man.
If he did retire to a monestary and died, he would have gotten a good christian burial and thus his ghost beyond hermetic magic and control.