Not s specific brand of camera type question and you'll get more responses asking on a general forum.

When photographing entertainers under spot lights, there is always more light on the subjects than you think. So the best thing to do (isn't it always) is expose for the highlights. If you can do a general spot metering of the area you want to base the exposure and then do a bracket around that setting. If you don't have a spot meter, just walk up to the stage and take a reading with your camera set at either spot or center weighted. Then back off and shoot. Black and white should give you whatever latitude you need to get enough good results to please the clients. And the fact that they've asked you to do it will make moving around to get your meter readings easier.