I have never flown as a courier, but several years ago I did some research on it for the same reason you are considering courier travel: mileage runs. I believe that in most instances, your overnight plan will not work. You may have to stay at the destination for a fixed amount of time because the company needs you for transport in both directions. In the Washington Post article, the trip was for 3 days, but I have found that most trips are 7, 10 or 14 days.

The companies find it a bit too much trouble to allow couriers to select their own return dates. It's more convenient for them if they select the return dates. This was the primary impediement to me for courier travel. I had no problem complying with the carry-on luggage rule, and no problem traveling as a solo passenger, but the rigid return dates made courier travel for me unattractive. They might work better for a true vacation, but not a mileage run.

Of course, if you are truly interested, do some research. You may get lucky and find a courier company that can be flexible on the return date.