How unsafe is a one-day megadose of vitamin a, taken to help knock out a nascent cold? (24, 000 i.U., then 24, 000 8 hrs. Later).

Not helpful. Vitamin a has not been shown to help with the common cold (uri). Excessive vitamin a can cause pseudotumor cerebri and consequent blindness.
Not likely to help. A one-time dose like this is unlikely to kill, but also unlikely to help. It is an entirely unnatural amount of vitamin a to ingest, and there is some evidence that taking vitamin a supplements could be harmful to the heart and the bones, and could increase risk of cancer. This would be much more likely for daily use than from one-time use, but not worth the risk in my opinion.
No harm. These dosages are not going to cause any harm. However, there is no data to support the use of vitamina as a supressant of a cold or upper respiratory infection. You are supporting the supplement industry but not yourself.