A similar function was served by the old Rhodegear "Flickstand" which mounted on the downtube and was spring loaded to normally stay above the tire but could be flipped down to hold the tire in position when leaning the bike against posts or carrying it. (But it wasn't designed to work with fenders.) Here's an eBay listing:
http://cgi.ebay.com/Flick-Stand-By-R...08185006r15064