improvise callbacks in plpgsql

hello performance minded administrators:

We have recently converted a number of routines that walk a bill of
materials (which is a nested structure) from the application side to the
server side via recursive plpgsql functions. The performance is
absolutely fantastic but I have to maintain a specialized 'walker' for
each specific task that I have to do. It would be very nice and elegant
if I could pass in the function for the walker to execute while it is
iterating through the bill of materials. I have been beating my head
against the wall for the best way to do this so here I am shopping for
ideas.

A simplified idealized version of what I would like to do is
begin
select (callback_routine)(record_type)
end;

from within a plpgsql function. I am borrowing the C syntax for a
function pointer here. The problem I am running into is the only way to
do callbacks is via dynamic sql...however you can use higher level types
such as row/record type in dynamic sql (at least not efficiently). I
could of course make a full dynamic sql call by expanding the record
type into a large parameter list but this is unwieldy and brittle.

Any thoughts?

Merlin