pg_dump custom format specification

Someone has some kind of specification or description of the pg_dump
custom format?

What I'm trying to do is an utility to remotelly syncronize postgresql
dumps, using lib_rsync to syncronize each table independently and
copying blobs only when sizes differ. 

I've made the same using the Tar format, but I think most of the
features and optimizations like paralell-restore are now only available
with the custom format.

I will do the script in Python, so, I need to know how to parse and
write pg_dumps in custom format.

Thanks in advance.

-- 
Guillermo M. Narvaja
Lambda Sistemas S.R.L.
www.fierro-soft.com.ar
Tel: (5411) 4139-0493/4
Cel: (5411) 15-6783-4435
Email: guillermo.narvaja@fierro-soft.com.ar
MSN: guillermo_narvaja@hotmail.com
Skype: guillermonarvaja
Lavalleja 519 1er Piso - Ciudad de Buenos Aires - Argentina