To add a user attribute to an object, we add it via the Edit Parameter Interface.

Add a parameter to the User tab under either the Attributes or Primvars section in the RenderMan tab. The parameter must be located within the User folder and must be prefixed "ri_user_" to be read. 

Setting up MatteID

Create a color attribute called "ri_user_MatteID0". 

This will emit the following to the RIB.

 Attribute "user" "color MatteID0" [0 1 0]