gpg has --show-session-key and --override-session-key.

While they're stated as being useful for key escrow (yuk!) they're also useful
for other things, like (a) the ability to retain access to specific encrypted
messages after destroying your own secret key material, or (b) to provide
accelerated access to a large encrypted mailspool by storing a protected
session-key index.

It would be good to expose those features in gpgme (though i haven't
internalized the API well enough to know how to do it).

I'd prioritize --show-session-key before --override-session-key, if for some
reason one of them is harder to do than the other.