Hello Mike. Wish you Happy New Year ! :-)

> If you really need to know if all *other* devices are outputting high
> except one, a possible method is to sense current flow from the bus to
> the OC pin with an external discrete transistor:
>
>     +5V----|
>       |   Rp
>      Rp    *-------PIC Sense In
>       |   |/c
> Bus---*---| NPN
>       |   |\e
>       |-R1--*------PIC OC In/Out
>
> R1 provides a path to guarantee that OC In reads low when the bus is low.
>  It should be about the same resistance as Rp.
>
> If some other device is outputting a low (holding the bus at ground
> potential) the transistor will never turn on.  Sense In will read high.
> If no other device is outputting a low except the PIC, the transistor
> turns on, and Sense In will read low.  Sense In will read high when the
> PIC is outputting high in either case.  While the PIC is outputting high,
> the OC In/Out pin can be read to determine the level on the bus.

There are also another idea how to make open-drain with only one PIC
pin.
Let us look at diagram below:

1. PIC pin is set as input
1a. Bus=HIGH => PIC pin=HIGH too (transistor in active mode)
1b. Bus=LOW => PIC pin=LOW too (transistor in saturation)

2. PIC pin is set as output
2a. PIC pin=HIGH => Bus=free (transistor is off)
2b. PIC pin=LOW => Bus=LOW (transistor in saturation)

           Bus
            |
            |
          |/c
 +5v--R1--| NPN
          |\e
            |
            R2
            |
       PIC in/out pin


R1 & R2 resistance resulting from Bus specification and
type of transistor and may vary from 5KOhm to 500KOhm .

WBR Dmitry.