I know RS Form provides the ability to add custom code and scripts to your forms.
Regarding JSignature, I didn't read all the API info from the link you posted, so I can't be sure how it works and its requirements.
Having said that, here is how I would approach this.
First of course I would read the documentation to fully understand how everything works and what I can do with it.
Then, I would experiment on the implementation, by following the main instructions on how to set up JSignature in any other page. - At the end, everything comes down to the basics (html, js, php etc).
So, set up the form, create the elements for JSignature and give them the required ids. I saw that it asks for a div element, but it is possible that it could work with other elements as well.
Of course, we need to link the JSignature scripts to the form. If the div is essential for the script, then we could add that special RS Form element, that is used to display static text and try to add the desired markup in there. It could be handy, to set the Form layout creation to manual, and build the form mark-up the way we need it.
However, at the moment, I have no idea at all about how all this will work to store the signature in the database, or give us the ability to catch the signature from each submission in any other way.
Just a guess: Maybe we could have a hidden file field in RS Form, and with a few bits of code, using the script's API and the RS Form ability to save in the database, we could get close to that.
Also, I would get in touch with the RS Form developers and ask for their input. It could be possible that they like the idea and implement such a feature in the coming releases.