I need to do the aggregate functions in c not pg/plsql so transition type needs to be accessible in c. If there is a way to do that without explicitly defining the type in c then that would be perfect.
Is there any particular reason why this needs to be done in C? Why not just create the type normally as per:
CREATE TYPE my_transition_type AS (a bigint, b text);
And then create your aggregate function using that type?
I can't help you with the C stuff, but you may be making far more work for yourself than you really need.