load are global builtin functions, and GDScript doesn't allow global functions to be defined, so no, you can't override them in the strict sense.
preload is quite hard to override without losing the parse-time feature of it.
However, you can "override"
load...by not using it.
Just write your own load function, which may wrap a call to
load or your custom mock resources, and call that one instead of
load directly. You can do this by creating an auto-load singleton, which you would call like this:
var resource = MyResourceLoader.load("some_asset.tscn")