have ran riding lawn mowers with belly decks through some pretty tall stuff before.
raised deck as high as it would go. put on "straight blades" non of them fancy mulching blades or high lift blades that have curves within the blades.
any were from 1/4th of the deck, to 1/2 the deck with the discharge side opposite side that the weeds were coming in at. with the plastic shut raised on the discharge side.
always keeping a keen ear out, for any "squaling" blades have been tangled up in weeds to a dead stop, and belt is just squeeling and burning itself up on the pullies. a good razor blade, a couple screw drivers, pair of pliers, and a pair of channel locks, to remove the build up around the blades. and off i go again, in the lowest gear i can go, and at the highest RPM's of the engine i can go.
p.s. i do take of the "shielding" on the mower deck so i can see the pulleys directly, that the blades attach to. so i can constantly take a quick glance to see if one of them have stops. there is a high risk of stepping off, or putting your foot off to the side and saying by by to your foot in doing this... or having a shoe string getting caught up in things.
but... there are some things i simply could not deal with...
you should be able to goto some local "equipment rental store" look up in your phone book "equipment rental" and you should be able to rent a walk behind self propelled, weed whacker (think miniture rotatory cutter / bush hog that would fit on back of a tractor) it will take a little bit of time, but you would atleast get things cut down to point were regular mower should be able to take over.
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put an ad on craigslist.org asking for someone, or call up some local lawn care folks / landscape folks. and get some bids, to see what it would cost them to clean it up initially.