In my book, deliberately reversing an engine on fire is not exactly sound airmanship. Reverse is used to help decelerate the aircraft during a landing run and is most effective at high airspeed especially if normal braking efficiency is affected by a wet surface.
It is quite possible that due to the urgency of a rejected take off caused by a engine fire warning, pilots will grab a fistful of throttles and pull all reversers hard back even though one engine may be on fire. Such is the excellent braking efficiency of current wheel braking systems, it takes only a few seconds of ground roll at max braking for the aircraft to have decelerated into ground speeds where reverse thrust is not very effective anymore.
According to the B737 Classic FCTM, there is only ten feet difference between effect of reverse thrust engine-out RTO, brakes, spoilers and one thrust reverser and effect of reverse thrust all-engines RTO, brakes, spoilers and two reversers. In other words practically no stopping performance difference.
That would suggest there is no advantage of reversing an engine on fire as part of the rejected take off procedure in terms of a quicker stop. On the other hand the disadvantages really stand out. Primarily the danger of spreading the fire via the reverse plume especially at low ground speeds.