I'd use the 12F675 (or possibly 12F629, which is the same as the 12F675, except doesn't have the A2D converters) - the 16F84 is significantly larger than needed (only 2 of the IO pins are required), and also needs an external oscillator.

10F20x doesn't have an INT input, so I'd discount them (although there's nothing stopping you from polling an IO pin instead of waiting for the interrupt signal).

I've got some spare 12F683s from a project I didn't need them for - they're similar to the '675, except have a PWM output capability (I was going to use them for a fan speed controller).

I'd be willing to program one up and send it on - but I'd feel a little uncomfortable doing the code without knowing exactly how it's meant to work (willing to discuss it though).