psql substitution variables

can anybody think of of a way to sneak these into dollar quoted
strings for substitution into create function?

would i would ideally like to do is (from inside psql shell)

\set foo 500

create function bar() returns int as
$$
  declare
    baz int default :foo;
[...]

that would eliminate the need for using the c preprocessor (or similar
tool) in some cases that require substitution in that way and (best of
all) allow psql to grok pre defined constants defined externally.

merlin