For signing this is already possible by entering the id of a specific signing subkey in the format "0xdeadbeef!" in the openPGP identity settings box. However, it's not possible to select a specific subkey in the key choosing dialog boxes (for the identity or for encryption of mail). There is no way to select a specific subkey for email encryption, not even with recipient rules.
Support for explicit subkey selection would help the implementation of concepts like https://blog.dest-unreach.be/wp-content/uploads/2009/04/pgp-subkeys.html.
Maybe it's possible to extend the key selection dialog such that it shows subkeys in a tree-like structure.