You can powder-coat almost any materials that don't melt in an oven at temperatures of 200° F to 400° F. Most shops prefer to coat metal items since that process is easy with the standard electrostatic application method.
Some non-metal items like ceramics, medium density fiberboard (MDF), and glass can be powder-coated too using different methods. However, materials like rubber, wood, most plastics, and resin can't be powder-coated since they melt more easily.