Hi,
to be honest, I have not the least idea why you'd like to have this option.
If somebody doesn't want to proceed, he either closes the browser (all given answers are saved unless you display your survey "all on one page")
or he selects the link "Exit and clear", where he expresses his will to have removed his given answers.
Well, you can hide this link that somebody can only leave a survey by closing the browser.
And you have to decide in your analysis if you use this responses or not.
Joffm
Anyway, as I said before you may use the startpage.pstpl to include this button.
Using your provided template you see this (with a little bit of css)
This is only the display.
I did not test if it works.
In my opinion each student who feels unhappy with the university's IT department should be able to license the basic version for one or two month (an investment in the future).
With the actual student discount this is really not too much.