Hi Hank,
As far as I know all I/O pins have internal clamp diodes, Vss->Pin->Vdd. As long as the top end resistor of the divider is large enough to limit the current going into the pin and up thru the diode to a safe level you should be fine. Use a logic level MOSFET and "disconnect" the bottom of the divider from GND/Vss.

If you look at the SERIN command in the PBP manual they are actually using this scheme, a 22k resistor in series with the TX-line from the PC makes it work just fine even though the TX-line toggles between something like +/-12V or there abouts.

Give it a try.

/Henrik.