deferred check constraints

Right now, it would be nice if I could get a check constraint to be
deferred.  Its a long story.  I want a circular constraint.  The way
things are set up right now, it would be easy if I could defer my
check constraint.  I'm doing a polymorphic relation.  One direction
is a simple reference a fixed table.  The other direction is a
reference to table that changes based upon the type of the item.  I
can do this check in a function which implies it is a check constraint.

I may figure out how to flip everything around so that the simple
reference constraint could be deferred.  That is one option

The other option is to add deferred check constraints to PostgreSQL.
I've never looked at the PostgreSQL code but I like parsers, etc.
How hard would it be to add this to PostgreSQL and is it something of
general interest or am I somewhat lost in the woods?

Thank you,
Perry