Hello John.

> I don't think there are any particularly good/reliable approaches for
> detecting more than two buttons with two port pins that can deal with
> combinations of buttons being pressed.  The closest thing I could
> suggest would be wiring buttons with different resistors and using an
> RC-timer based approach; not sure that would be 100% reliable, though.

Why not? There are method with additional external shift register.
It have excellent reliabilty (i practically use it).
First PIC pin clock the shift register ( D input through the low pass RC
and \C input directly ) I post idea to PICLIST some minutes before.
Register with open collector's outputs. The keys wired directly to
register output.

Second PIC pin read the second common wire of key's matrix.

I apply this method to scan up to 16 keys matrix with two PIC pins.
(One additional shift register chip + RC)

WBR Dmitry.