It is possible to manually toggle sign for a message, I wonder how it can be done for any message created?
(setq wl-draft-config-alist '((t (mime-edit-pgp-processing . '(sign)))))
See Section 7.1.4 of the Wanderlust manual, Dynamic Modification of Messages.
I would simply create my own function that signs and sends, and use that instead.
(I do not use wanderlust and therefore do not know all of its possibilities).