In my many years as an art teacher I have had a few kids with broken arms. Some even come in with a grin expecting they get to sit and do nothing. This is what I have always done different with them...
They create with their able hand as a form of physical therapy. I tell them that their grade will be based ONLY on their level of participation, NOT the quality of their work. If they try, they will not fail. I don't sideline my multiply disabled students, so it's similar, their disability is just temporary.
DON'T make more work for yourself. Let them participate at the level they are able. They may find out they have a hidden skill. At worst, their coordination gets better with their non-dominant hand.
If they are in a 3D class where two hands are necessary, then maybe an aide can work with them, or they can do a drawing version of what you are doing in 3D. That said, even with clay, I have them pinch small pieces, glaze what has been completed, and do what they can with their able hand.
For those that flatly refuse, feel free to assign book work, a research paper, etc., and I bet they will give project work a try.