Set the zone to End Of Line resistors, place the end of line resistor on the zone input, and put the Open Collector output device on the zone side of the end of line resistor, not on the negative side. When the Open Collector device shorts to negative, it will violate the zone input.
I now understand how to wire up Spanky's suggestion, I think. I have a GE Quik Bridge 8016 Loop Receiver which has eight OC outputs which I use for wireless water leak sensors. I didn't consider looking at it earlier as an example because it only has one terminal per OC output, so I figured it was different. Now that I know this new OC output board's "2nd terminal per output" is just a convenience connection for shared grounds, I realize they are logically the same with regard to OC outputs.
The Quik Bridge is powered by the P412 PS, which has a shared ground with the M1G PS (via a connection to the ground terminal on a DBH in the same box). There is only one wire per channel/zone between the Quik Bridge and the corresponding M1XIN zones. According to the Quik Bridge manual, this is a non-supervised loop connection, and the M1G zones are set to Normally Open. The manual shows two additional wiring diagrams, both with EOL resistors in them, and both with a single connection from the OC output to its corresponding M1G zone positive terminal. The "supervised N/O loop" has the EOL resistor in parallel, i.e., directly between the zone's positive and negative terminals. The "N/C Loop" shows the EOL resistor in series with the positive connection between the OC output and the zone's positive terminal, with nothing directly connected to the zone's negative terminal. Since the OC output board will be located within a few inches of the M1XIN board, I don't know of any reason to use EOL resistors, although if I can find some laying around, I guess there's no reason not to use them.
Ira