I've seen no direct proof one way or another on whether Resin was used as a stunt saber for duelling in ROTS. There have been snap shots of Mace's and Anakins with parts all strewn apart, as well as Nick Gillard explaining how teh graphite blades were attached via set screw...I don't think resin would take that kind of abuse since teh blades were inserted into the hilt and then set screwed were tightened to hold them.
As far as any good photo and video I have seen of the stunt sabers they were all metal...
However teh regular stunt sabers, the falling props, the ones that are worn in running etc. where stunts need them to have the saber, those ones are usually rubber or resin.