Hi,
glad you ask for this.
The forms now include just the very basic elements because we were waiting for users' interest in other fields.
It's definitely on our roadmap to improve this feature based on the user requests.
I filled the signature field requirement to our backlog now
https://github.com/jsreport/jsreport/issues/754
If you are interested in the contribution you can look here at the main code.
The formElement helper implementation doing some validations is here.
And mainly the tests are here.
I was developing this in a way that I mainly prepared the desired pdf with one form field using acrobat.
Then inspect the pdf structure using rups tool.
While also reading the pdf specification.