I don't think a reasonably sized reservoir feeding from eg. wastegate pressure pipe would run out that fast, as each pulse would replenish it; a delayed jet engine cycle, if you will
I think separate runners to cylinders 1-4 and 2-3 have something to do with it; conserving air os something else. It can't be from a bypass, since those aren't used in rally cars.
But, whatever you do, use antilag very very sparingly... it'll eat your turbo in no time. Mine works simply by feeding a rich mixture into a cylinder and then skips a spark. When the next spark hits the cylinder, it's on the exhaust cycle.