Okay, I just found this on a Microchip forum (to quote):
"Any PIC I/O pin can be open drain.

Let's say you are using PORTD bit 2. COnfigure this bit as an input (initially). Set the output PORTD bit 2 = 0 (or port for a 16F). When you want the output to be active low then instead of writing to the lat or port register, you write the corresponding tris big. But toggling tris you are switching the pin to input (open drain) or output low."

So, it's basically suggesting that any port with I/O functionality can be "simulated" to be open-drain. I'm guessing this is what Pic Basic does ie. toggle the TRIS register to pull a port low?

Cheers,

Troy