Which are the input/output requirements of a 12V trigger interface?
Best hint I could find:


  Note: The VIC-12 requires a 100mA current from the projector trigger. It is not compatible with 20mA projector triggers used on some Sony, Panasonic, and Yamaha projectors.


So, I guess, your output should ideally be able to deliver 20 mA or 100 mA, at least (and probably not much more to protect faulty devices).

Then, I also found this "schematic" (look for the links on the right of the page) of a projection screen roll-up/let-down trigger:



My guess is that the capacitor labeled with 12V actually should be the trigger input, followed by some Low pass+voltage divider that have an effective DC resistance of 9.4 kΩ.

Since that thing (obviously designed by experts who totally didn't copy and paste together that schematic in photoshop!) probably reliably works, I guess you might be overthinking this – and that there's really no standard to this.

I do agree, it's not hard to come up with a receiving circuit, but seeing that there's really a lack of any usable link found via Google (and seeing that stackexchange Q&amp;As rank high automatically), I'll take the chance to describe what I think would work, for future readers:

Build something that has 500 Ω to 10 kΩ input impedance (I guess it doesn't really matter, seeing that there's neither any chance for the signal to be of significant speed nor of your detection to interfere significantly with the power source), and just hook it up in parallel to the trigger port and any other devices to be triggered from the same signal.