suggestions for lifting when arm in short cast

On the assumption that the client has been cleared to do any weight lifting:

I would use a cuff to attach around the wrist and then do exercises on a cable machine (or tubing/dynabands).

I also use this modification with a client whose hands are very arthritic, and she cannot hold weights very easily. I do not know what you have available; at my studio, I use this technique also on the Bowflex and on the Total Gym Power Tower.