Remove a recipient from a message without viewing it

[Nathan Krasnopoler]

Is there a way to remove a recipient from a message without having any
of the private keys needed to view the message?

For example, is message M is encrypted to Sam, John, and Bob as text
C, I would like to output C_s that is M encrypted only to Sam, C_j
that is M encrypted only to John, and C_b that is M encrypted only to
Bob.

If this is possible, I think it would also preserve any signatures on M.

I would prefer to do this in a python interface, but a shell/gpg or a
C interface is also workable.

-- 
Nathan Krasnopoler