Ok so I've gone to have a look and it's not as straightforward as I first thought. Basically the ignition timing and the idle airflow are both in closed loop control at idle. That means that if you change them to an incorrect value they will self correct in just a couple of seconds. I would need to look further into the tune file and see if there is a way of having either the ignition timing or the airflow not be controlled at idle. This isn't a common thing to be looking for, so I would have to sit down and browse through a few hundred, or possibly a couple of thousand parameters to find a likely candidate, then I would need to flash my car etc etc.
While it doesn't cost me anything to do this, it does take a lot of time. This certainly isn't a request you would take to a workshop, as you could easily run up a few hours, or a whole day sorting it out.
I'll have a look through the tune when I've got some quiet time during the week, I wouldn't hold my breath for an outcome though.