postgres_fdw and defaults

I know we've discussed this before, but I have just had the unpleasant
experience of trying to get around the difficulty of inserting into a
foreign table with a serial field, surely a common enough scenario that
we should try to deal with it better. The solution of using a local
sequence really doesn't work, as there might be multiple users of the
table, as there will be in my scenario. I opted instead to get a value
from the foreign sequence explicitly before inserting, but that's pretty
ugly. So I am wondering (without having looked at all closely at it) if
we could set an option to tell the FDW that we want the foreign default
to be used instead of a local one. Is the difficulty that we don't know
if a value has been explicitly supplied or not? Maybe we could have some
magic value that we could use instead ('foreign_default'?). I'm just
throwing out ideas here, but this is really a wart that could well do
with attention.

cheers

andrew