In a way, although I can see why you might want the functionality, I'm struggling to see why you would want this functionality in the specific examples that you're giving.
Your trees and gates would surely be non-conspicuous (i.e. invisible from the object list) scenery items, and would be mentioned instead in the location description... "You are standing in a forest, surrounded by trees."... in which case whether they are created before or after the object list is printed doesn't matter.
If you really want "object list visible" trees in each case, then I would suggest that creating multiple "dumb" tree objects would be the way to go, because it would be probably just as easy to define the interactions with each "tree" based on the location range/zones.
But yes, Adventuron doesn't seem to give you a way of doing something between printing the location description and listing the objects, like a system such as PAWs does.