It's a little tricky to do that with SignNow's "Send for Signature" button, as it appears to include multiple modules at runtime when you click the button. A lot of the code is hosted elsewhere.
I did however find a feature in the SignNow SuiteBundle that lets you use Advanced PDF templates built for other record types, so we might go with that if we don't relent and just auto-create an Estimate for each Opportunity.