Dear Concert Presenter:
I have a couple of suggestions for what to do when the guest conductor shows up for rehearsals on crutches.
First of all, you could put something in the program or have an announcement made about just what his situation is. Very likely it's temporary, and it's also something that everyone in the audience will notice if they have normal eyesight. We were all very curious.
Next, do not make him come on stage and go offstage between all of the pieces, especially if there are three works on the first half of the concert. I do realize that very possibly he decided himself that he would do this; maybe he needed a few minutes to shift his thoughts from one piece to the next. Maybe you could encourage him to just stay on the podium, even though the stagehands were busily moving soloists' music stands around.
Lastly, how about a podium with steps? Watching him hop onto the Zellerbach podium was scary: we were all hoping he would not wind up in the laps of the first-row patrons. Or, worse, see season 2 of Slings & Arrows.