I don't recall ever saying "KM can't do that," because I usually can find a way, but for that statement I don't think it's possible. There are many people here smarter than me, and maybe they can think of a way to do that. Basically, it sounds like you are trying to replace the KM Engine's mechanics with another kind of layered engine to override the real engine's mechanics. That's asking too much.
What I believe is possible is what you describe as not preferable, which is "adding a ton of hotkeys to the trigger conditions of a 'frontmost window validation' macro". Maybe then.
In any case, I have a question. When you talk about hotkeys, are any of those hotkeys "regular characters" like A-Z, or are they simply limited to function keys? I don't think you specified. That might make a difference to the solution. This is important, I think.
I have a totally different approach that might satisfy you. I'm trying to think outside the box. Instead of trying to solve the problem by going to a ton of work to interpret function keys, why don't you just have an annoying noise or beep sound off whenever that Properties window appears? That's how I would address this issue. I think that can be done with perhaps a single action in a single macro.