For quite some time, I wanted to use subkeys for signing at some places where I do not want to have my certification key stored, however I trust this location enough to have signing key available. I knew it should be possible with subkeys, however I never found time to look at it.
First I needed to add new subkey which will be used for signing:
gpg --edit-key 42B7511D > addkey
Now choose 4 (RSA for signing) and wait until new key is created.
The only remaining step is to transfer subkeys to the other location:
gpg --export-secret-subkeys 42B7511D > /tmp/key-sub.gpg scp /tmp/key-sub.gpg other.place:
Import them there and deleting any unwanted (eg. encryption) subkeys:
gpg --import key-sub.gpg gpg --edit 42B7511D > key 1 > delkey
And it's done ... much easier than I expected :-).
Hmm. Do I understand you right? other.place is the host which you don't trust? But you copy a subkey there to edit it THERE afterwards? Not a very good idea on an untrusted location, if you ask me.