I think for it to work best, Stewart would've had to do an American accent-- otherwise he would just sound like, y'know, Captain Picard. except swearing a lot. but I've never heard him do an accent other than his own.
There's some logic to casting Patrick Stewart as Spider Jerusalem that's always sailed so far over my head that I didn't even notice it was there. I have friends who swear by the idea, but it seems like the most unlikely thing in cinematic history to me.
But after seeing Robert Downey Jr. in A Scanner Darkly, I have trouble imagining anyone else in the role anyway.
when I was in school, I worked Tech (lights and sound) for our on campus theater, and he came and did his one man "Uneasy Lies the Head" show. He was wonderful to work with, very nice and very geniune, even though it was a bazillion degrees in the place with no AC.
I do wish I had heard him say "fuck" that would have been great.