Whoops, sorry, I should have put that button in under the Commands menu. Which you could do but I'm guessing recompiling a C# dll is beyond what you'd like to do for this
PlayerData/inputmappings.dat _is_ the right place for changing the keys for an existing bind. But you can't define new binds there.
Since this behavior is already implemented by the main input handler, though, you can add a bind to that existing behavior very simply:
1) In GameData/Configuration/InputAction/KDL_InputActions.xml
2) Add this at the bottom (before </root>) :
<input_action name="ScrapUnits"
display_name="Scrap"
description="~*~"
dll_name="AIWarExternalCode"
type_name="Arcen.AIW2.External.Input_MainHandler"
default_key="Delete"
>
</input_action>
This will be overwritten the next time we update that file, but that bind will probably be present in our update. If you wanted to make a similar one and avoid the overwriting you could put it in a mod in the XMLMods folder.