What do you need a comparator for? You either have 0V or your divided supply voltage. For a wide input range, use a transistor to invert the signal. And 5A does not need a particularly large FET, SOT-26 options exist for that sort of current. Bulky diode, though.
E: SOT-26 options exist in NFET.. checking for PFET.
E2: Yes, many choices in SOT-26, SOT-23, cute little DFNs (2x2mm small enough for you?)..
I really tried to understand how this works before I posted a reply, so I wired up some tests, but I don't understand how I can get an indication voltage that I can use...
The image I'm attaching is a MOSFET being used as a "diode" the way it would be used for reverse-polarity protection. I'm using an IRF540 here, but I also tried a couple of other mosfets and they work the same. I understand that they're not modern mosfets, however, and that there are specifically mosfets designed to be used this way.
Before I connect a load to this, the source of the mosfet is 0V, and when I connect a load it's around V0.001, so it's behaving like a very low-value resistor.
How would I directly use this behavior, without a comparator, to generate a voltage high enough to turn on a small signal mosfet? I assume that I'm misunderstanding what you meant, but I don't know how else I'd configure this...