I've played around with the plants database and I think the answer to my question is No but in case I've missed something, is there a way to search for plants that will grow in a specific hardiness zone?
The Minimum and Maximum Zones fields only return results where, e.g., Minimum = search value, rather than results where Minimum <= search value. So that means running separate searches for all of the zones at or below the zone I want, which is a lot of searches if other criteria are specified.