I would be interested in that myself as well. I was going by the FSX SDK. As for the aircraft.cfg, I only found this sort of thing (found in the default A321 aircraft.cfg):
exit.0 = 0.4,  45.50, -6.0, 7.0, 0  //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
I do not see any mention the jetway operating speed within either of them. So if there it can be done in the aircraft.cfg, that is great.
I have always found the jetways to be the same speed regardless of the aircraft I am using. But maybe I just think they are.

Mark