[mattthebaker]

You can draw a small amount of current from the logic signal in high state and store it in a small amount of capacitance, a low enough power device would be able to operate off of it. There is a class of sensors which use and interface called 1-wire which is ground and signal/power (2 pins).

You don't need a very accurate clock (quartz) for ~100 Mhz SPI. A ring oscillator is just a chain of inverters. If you want to match the bus frequency accurately you can train a PLL against the signal clock.

Not sure what the point is about the pullup, to flip the bit you need only to overpower the output buffers of the other chip.

Cramming it all in a 0402 would be a stretch but I could see it being possible with the right resources. Having only 2 pins would be pretty useless if you can't snoop the address though. And modern NOR devices on a motherboard is almost certainly using the high speed 4 pin serial interface rather than standard SPI.