If this was enough to temporarily replace breathing I wonder how that would feel if you were otherwise healthy. I imagine not breathing would instinctively feel quite strange and even distressing.
It would be quite distressing because of the accumulation of CO2 in the blood, even with completely adequate oxygenation delivered intrarectally. The slight change in acid-base balance is what makes a person feel the need to breathe, and CO2 is an acidic byproduct of metabolism. This is why people with metabolic acidosis (e.g. in diabetic ketoacidosis or sepsis) have an increased respiratory rate.