Are you sure the Digital Signature Log isn't updated? Make sure you select the correct Application and Signature Object in the selection screen of DSAL. At least interface IF_DS_RUNTIME updates the log, even for non-standard strategies/applications/objects.
Update: I checked the function module SIGN_SIGNATURE_CREATE which isn't a BAPI and it's not even released. It appears to be a low level API, I suggest you look into using IF_DS_RUNTIME instead.