Hanging them from a cord or chain is a good idea! No taking up shelf space that way.
Assuming the pumpkins are made from styrofoam or polystyrene the melting point is something like 460 degrees F. Unless you're living on Mercury I wouldn't worry. Of course it would be fun to get out the mini blow torch and make some sort of giant mutant pumpkin. I know my husband would then be more into "carving".