Passing fdw_private data from PlanForeignScan to PlanForeignModify

I'm currently implementing DML actions in the informix FDW module and
trying to understand how UPDATE/DELETE actions interact with the various
structures, especially how states between the associated ForeignScan and
ForeignModify actions could be transmitted. As far as i understood, with
e.g. an UPDATE action on a foreign table you get the following sequence of
callbacks:

GetForeignRelSize
GetForeignPaths
GetForeignPlan

PlanForeignModify

BeginForeignScan

BeginForeignModify

<repeats>
IterateForeignScan

ExecForeignUpdate

</repeats>

EndForeignScan

EndForeignModify

During the planning phase, the Informix FDW currently plans a server side
cursor, which identifier i'd like to pass into the planning phase of the
modify action to reuse it in conjunction with UPDATE ... WHERE CURRENT OF.

I understand that the ModifyTable structure passed to PlanForeignModify has
a list of associated plan nodes, from which i can access the ForeignScan
plan node associated with the current modify action, thus having access to
the fdw_private data generated during the planning phase of the ForeigScan
node. However, it's a list and i currently don't understand on how to
reference the associated ForeignScan node reliably, given that there are
cases with more than one node in this list.

Any pointers, someone?

--
Thanks

Bernd