No matter how fast a conveyor travelled it would NOT stop the thrust from pushing the plane forward, unless it could excede the max. speed of the wheels or the wheels had brakes applied.
The trust would still push the plane forward, just as the thrust on a Harrier pointed downward is able to lift even if you took your hypothical conveyor and made it vertical and put the wheels of a harrier on it's nose the conveyor would only spin the wheel. the harrier would still climb because of it's thrust.

Not trying to offend