Serial Communication over 1 Pin?
I am not a PIC person, but normally for serial standard you need three lines:


Send (TX)
Receive (RX)
Ground (GND)


Since you dont need RX, you could use the circuit ground and use pin 3 for TX, if your Arduino and your PIC agree on the baud rate, and other parameters related to serial comms (stop bits, parity bits, etc).  This in theory could be standard serial comms if you can get your PIC to do it.

SPI is not going to be possible, since SPI requires an independent clock line.

Another thing to consider would be Dallas 1-wire protocol, however, in practice it will be tougher to implement than either real or hacked serial.