I don't use keyswitches live, but instead plug them in after recording the MIDI track. Consequently, I could use just about any controller.
Futhermore, I usually apply the keyswitches to a separate MIDI track from the main performance. On the rare occasions when I do need to "play" the keyswitches in real time, I can use any controller because it's still a separate pass.
Another advantage to putting keyswitch events into a separate track is I can easily transpose them so there isn't so much white space between them and the notes in the PRV. It's a lot easier to program keyswitches if you don't have to scroll the PRV. It also makes it easier to stack keyswitches for multiple virtual instruments on one screen, e.g. violin/viola/cello/bass.
There is yet another option, which I saw Jordan Rudess using in a video, and that is to pick up a separate keyboard controller just for keyswitching. I've got one of
these, which I could use that way although mostly it's just for packing along on road trips. It's tiny - small enough to set atop my main controller, close to the pitchbend and modwheel.