I ended up changing my plans. Could not get anything remotely decent for a rental from a neighborhood location and for a dropoff at the airport. The T&Cs of the various coupons either excluded one-ways or excluded non-airport locations, etc.

So, I ended up instead renting for 2 1/2 days from/return to a neighborhood location using this code
https://www.hertz.com/rentacar/speci...jsp&Category=Y
$20 per day for a midsize (or $40/day for a compact!).

The neighborhood location in DSM works better (and allows after hours drop offs), actually, for our trip and the hotel we booked is better and has a shuttle -- so I'll drop off the car late on Sunday evening, get the hotel's shuttle to the hotel and take it to the airport the next morning (and not have to deal with returning a car at the airport early in the morning before our flight)