Steve
Without any description of the stain and what you think has caused it, it's verydifficult to provide a solution. Generally the stains on the thermoset plastic are surface stains and it would depend on the viscosity if they have become intergranular/sub-surface types. Bon AMI is actually an abrasive scouring powder just very fine. Water mixed with the bon ami creates a caustic mixture whick will penetrate the top layer forcing out some types of stains. The abrasive component opens up the grain structure and removes the surface staining allowing the sub-surface stain to bleed through.
I clean my Performer thermoset table with boiling hot soapy water and a scouring pad for stubbon stains. If they don't come off - well it is a BBQ after all.
Regards