Welcome to WebmasterWorld Guest from 184.108.40.206
If you follow the Adsense TOS you shouldnt have any problems at all.
But anyway, recently I made a change in my ad display that involved an image, I wasn't quite sure if it followed the TOS correctly so I decided to go to the source. I contacted AdSense support asking about this and provided them with an url where they could see an example of what I was talking about.
They replied to me rather fast and they were very polite, they told me that they appreciated that I took the time to ask them and told me that there was not problem at all.
I think your best chance, and what I recommend, is to make and example page of what you want to do and contact them.
My first impression to this practice was that it was a violation of ToS, but I am not sure....