OK, I give up.

[Stephen Satchell <list@xxxxxxxxxxxx>]

Implementing my new parametric firewall generator to interface with 
nftables.

So I don't need an answer to my question of using iptables rpfilter in 
raw/POSTROUTING.

Interestingly, the nftable translator gave me an answer when I asked it 
about the iptables statement with rpfilter in raw/POSTROUTING.  I think 
they call that a "clue".