Two years after the fact.....
No, I've not seen this, the early (no ricasso) ones have a thinner variety of leather buffer, and the later RZM types are with a thicker leather buffer.
Some of the ones I have are very hard and dried out with age, but would not be mistaken for anything other than leather.
Interesting question though, but I'm not aware of any knives produced with ersatz materials. I wonder if anyone else will chime in?
I have seen a rubber/plastic ersatz material used for the belt loop on some HJ scabbards but that's about it.....