My initial concern was that this would bloat up postgres_fdw with type conversion code, but the patch shows that it isn't that bad.
One additional thought - in an offline conversation I had with Robert Haas, he had taken the position that any FDW was likely pulling values across a wire protocol and then synthesizing postgres-looking statistics from that, so going with cstring inputs was fine, possibly preferable. I'm highlighting that we're now passing in NullableDatums in case he wanted to object to that change.
