One more thought, oversize the holes in the plates to allow for wood expansion/contraction. Maybe add a rubber washer so you can snug it up and still allow for movement.
Is there an epoxy that can adhere the FAP plates to the stone? So I don't need to route out a channel for the anchor heads in the underside of the bar top?
I think there are epoxies or other adhesives specifically for attaching steel to stone, I haven't done it myself but I would guess if you talked to a place that sells stone they could advise or talk to a place that sells epoxy and adhesives.