Seperated hasing and signing in PGP

Hi all.

I need to do a PGP signature in two steps, first the hash on one machine
and then the actual signature (signing) on another. The reason for this is
that the data that is to be signed is extremely large and cannot be moved
to the signing machine, which is off-line and only communicates through
a “sneaker net” (USB stick).

I have looked around I can only find Python code that either calls GPG
(which does not have this “two phase” possibility) or code that calls some
C library that will require rewrite to make this possible.

Does anyone know I could achieve this in Python?

Thanks,
Håkan