Since there isn't any responses yet
you ought to try the Lawn Mowers Forum
or maybe the Tractors Forum.

I have a field that got a bit overgrown and was told that a brush mower would work. After we had 6' saplings come up I decided to keep the field cut.
You could look up high weed mower - maybe rent one.
http://www.billygoat.com/site/intro.aspx?pid=19

We bought this mower for when it gets too tall.
It has back wheels that are bigger but not bicycle sized.
It isn't perfect but you have to tip the front wheels up and keep lowering the front down to cut the tall stuff.
http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100521267&N=10000003+90401+503347+1953&marketID=401&locStoreNum=8125