Dwight Freeney would abuse a sumo wrestler so much, the guy would quit in the middle of the 1st quarter. The QB would get destroyed play after play. Regarding the "tall guys blocking FGs"... this might work, but not with them on the line of scrimmage. Instead, you make them stand right under the goal post. If the crossbar and a basketball rim each are 10', a 7-footer should be able to get his hands about 12' or more in the air from a standing leap. Many long FGs barely clear the crossbar, so my guess is these guys could block their fair share of kicks.
(Yes, it is perfectly legal.)