Thanks, for the welcome.
(Shameless Plug, here) Book of Sires is set between the years of 439 and 484 with the addition of a Salisbury Anarchy appendix addition. It is possible to set up the exact scenario of a PK pulling the sword. After all, what determines the right to pull it? How did the sword get there? Is it the same as Excalibur? What happens afterwards?
If the gm determines the right to pull is blood, then who says Constantin or Aurelius did not have another unknown son? After all, Uther did. Could Uther have a second unknown son as well?
Back to the point, do people feel Ars Magica is broken or flawed because the main characters are mages? Perhaps they are put out because they want to play a thief. Well, you can in Pendragon, but it is not all that fun. But, during the anarchy, go ahead and set yourself up as a robber baron. If going by the book, the PC might be in a bit of trouble when Arthur comes around, but nothing says you can't be a "sheriff of N..."
Be creative, and you might find there is a whole list of things you can do.