Connecting through DCA to/from Republic E170/175 aircraft relatively routinely, I would say I wind up on the center pier maybe 20-25% of the time. I happens with regularity, but not a lot. They are using 4 center pier gates if I recall correctly, and they can use any mainline gate in the north pier but usually don't use the shuttle gates.

I would think they probably have either elevator access of some kind or escorted security reclearance for wheelchair passengers transiting between the two terminals. I know the elevator that goes up to the US club can also go down a level with an authorized airport ID badge.