Hi JWS,
There could be a couple of ways to approach this, but this depends on how are you are attempting to add your customer's signature. We have had this request in regards to mobile users in our forums, I found this link, which talks about how it can be done using a specific Android Application. I am not making any recommendation here, but the user that pointed this out had this working for them, it was a good idea and can maybe give you some others.
Out of curiousity, do you require the signature on the actual "Edit Sales Order" screen, or would it be ok to have the signature on the PDF document that will be created from this Screen? It would also be possible to print the Sales Order PDF, have the customer sign it, then add it back to the Documents section of Method after it has been signed. Would any of these scenarios work for you?
-Ben