Foul ball accelerates?

yes its possible in elastic colission....momentum is conserved in any type of colission.IF M is mass of bat with swing intial velocity V and m is mass of ball coming with velocity 90 then..

90m+MV=120m (if bat comes to rest after hit)

I think it is possible, but probably only if the ball is fouled straight back, AND up. If the batter just tips it and it barely changes course, it probably isn't going to accelerate at all. If it was popped straight up, it could (somewhat obviously) be going MUCH faster than the pitch. 120 sounds high to me, but if he fouled it back and up, it could definately be going faster than 90 mph.

I thought a line drive right off the bat went something like 120-140. It wouldn't be going as fast as a line drive.

The foul accelerates if the force of the bat produces a vector which adds to the vector of the ball to produce a vector with a higher value. remember vectors are scalier. It depends where ball leaves the bat.

in addition to the SOMEWHAT elastic collision of the ball with the bat (which would just reverse the direction of the ball), the momentum of the swing is added to the ball and thereby increases its' velocity

This is not possible, the ball must lose some momentum.