We did something similar-ish a while ago. We actually had access to a treadmill, but we weren’t allowed to wreck it or paint it. In the end, we decided not to use it, because we didn’t need a full length shot. Instead, with a bit of practise, out ‘talent’ was able to walk on the spot in a convincing enough manner. Maybe it’s an option for you?
Alternatively, there’s this?
Don’t add the handles, and paint it green??
Graham
http://www.qcit.com