I replaced my wiper arms with ones from Orielly's and don't remember any issue pulling the bushings or putting the new ones on. Didn't have to remove the shaft assembly.

Bill;

If you install the Bushing with the assembly on the car, there's not much other than applying impact with a Hammer, either manual or an airhammer would do it too I suppose...apparently this works...see link (K Bjorklund's picture): http://www.sw-em.com/Wiper_Arm_and_Blade_Notes.htm
...I don't like the hammer part so much either, but maybe having a helper backing up the shaft under the Dash with a bucking mass would minimize the forces we're both concerned about...

Good Hunting!