There is a checkbox at the bottom of each script window for running that script with Full Access priviliges. This script can be used to create new records, etc. when the user doesn't have such priviliges.
I am guessing but perhaps your signature capture doesn't work with accounts without the proper access to create and save records.
So, perhaps a button that performs a script and goes to a signature layout and has the box checked. Use a pause button after Go to Layout and then have a resume button on the layout to continue and save the changes.
Not sure if this works but it might.