Thanks Thom, for all your great help!
I tried this exact method several different ways, but never quite found the solution, that worked out with my specific workflow.
I ended up using the "button over" method.
A button over the signature field, (button properties set to no effect, no color, and no text), when a user clicks it, it runs through the same validations my Submit button does. If all comes back valid, the button hides, and brings up a pop, that then instructs them how to attach the signature, and follow through with submission. I've also seen this called a two-click method, snce the user is clicking twice to sign. the catch is they don't know anything about the validation unless it finds a problem, so it appears to be "informative" as to the signing, and submission.