customizing pg_dump together with copy.c's DoCopy function

My first ever newsgroup PostgreSQL question... I want to move data
between some very large databases (100+ gb) of different schema at our
customer sites. I cannot expect there to be much working partition
space, so the databases cannot exist simultaneously. I am also
restricted to hours, not days, to move the data.

I felt that pg_dump/pg_restore with the compressed format would do the
job for me. I was able to create a modified pg_dump program without any
difficulty. But I need to customize the speedy COPY FROM and COPY TO
commands to perform my necessary schema and data content changes. I
tried copying /src/backend/copy.c/h to my customized pg_dump project,
renamed them and their DoCopy function, and added it to my makefile.
This created conflicts between libpq-fe.h and libpq.h. For example:

postgresql-7.4.13/src/interfaces/libpq/libpq-fe.h:191: error:
conflicting types for `PQArgBlock'
postgresql-7.4.13/src/include/libpq/libpq.h:39: error: previous
declaration of `PQArgBlock'

Is it possible to compile-link together frontend pg_dump code with
backend code from copy.c? If not, how can I go about customizing
pg_dump to have low-level control over the speedy but inflexible COPY
TO/FROM commands? I already tried all this with regular sql calls and
it was unnacceptably far too slow.

thanks -Lynn