Electroless plating is similar to electroplating. The only difference is electroless plating does not require external biasing. Instead, it requires a reducing agent (some chemical). In this regard, it seems not possible to deposit other materials than metals or alloy via electroless plating.
Some papers describe of electroless coating of polymeric materials like PTFE in combination with metals like Ni. But i am not aware of coating only non metallic coating on substrates through electroless coating.
You can electroless coat a non-metal, though it's not often done. The only practical applications I can think of involve lead. As an example, if you have lead pipes or pipes soldered with lead, they will become coated with lead-oxides and over time this lead will leach into the drinking water. You can add chlorine or hypochlorite to the drinking water, you form a coat of insoluble lead chloride on the surface of the pipe, and soluble, non-lead oxides in solution.