I see your problem. I think it would be a nice addition to be able to add the actual material of the part as a parameter. However, setting up two studies to accomplish the same thing is probably just as easy. You can set up your first study with whatever material you want, make sure it meshes and solves and then duplicate that study and just change the material and rerun. If in the future you make any changes to the model than you can use the run all studies feature to automatically rerun both (or as many as you have set up) your studies with the different materials which would very similar to what a parameter would be like anyway.
Thanks. Duplicate studies is exactly what I'm going to do.
I've had problems with the "run all" command in the past. It didn't seem to remesh for each study, so only the first study solves. I'll have to revisit this.