"You do not need "free" i/o pins or serial pins to use ISP - add a resistor between rxd and rest of circuit, and same with txd."

The RXD pin is being used as an input that gets shorted to ground through the contacts of a relay when the system is idle. Unfortunately it is too late - the prototype boards have already been assembled and conformal-coated, so I cannot add any resistors.

Next time, I will make sure they do this during the hardware design.

Thanks for your reply,
Joe