"Assuming that power can be sourced from somewhere else, analyzing power consumption will not detect an implant."

Yes and that is a big assumption. I think constraining the space in the enclosure is actually the best defense against all classes of attacks especially ones that use external power supplies. It's also the simplest. Just pot everything and cram it in the enclosure like a sardine so there's no space to add parasitic hardware.

Sensing currents at the display level is so easy to beat with simple amplifiers. With a good amplifier stage the attacker could detect bits with mere nano amp consumption.

You'd have to make a physically secure enclosure for the display (can't access pins) AND make it encrypted for that idea to work. It's certainly possible.

I think choking the external power supply's access with volumetric constriction is the strongest and simplest. Then you can do detection as I described and yes hardening the display design all in conjunction.

With the proliferation of AI powered attacks, I think Apple will face more pressure than you think to do things like authorized signatures on data payloads otherwise no forwarding.