Thank you @Harold, a few more questions if I may. 
1.
You mentioned creating a small PCB, one that might even fit inside a camera trap’s casing. Might there be value in turning it into a generalised interface board between the triggering circuitry in a camera trap and external components like your acoustic trigger, other camera traps, stereo cameras, speakers, etc.?
So something like this:
┌─────────────────┐ ┌───────────────────┐ ┌─────────────────────┐
│ │ │ │ │ │
│ camera trap │◄───────┤ trigger interface │◄───────┤ │
│ │ │ │ │ external components │
│ trigger circuit ├───────►│ board ├───────►│ │
│ │ │ │ │ │
└─────────────────┘ └───────────────────┘ └─────────────────────┘
The board would expose pins or other ports to serve as a standard interface for external components to either trigger, or be triggered by, a connected camera trap. So, you would connect your acoustic detector to this board and cover the camera trap’s PIR sensor. And for me, I would connect another camera trap to this board.
In other words, this board would give a camera trap the generalised ability to accept expansions. Would you be interested in figuring this out?
2.
Also, based on what you’ve seen in the BoomBox documentation, do you think hacking into the PIR sensor circuits in a camera trap is a difficult job? I used to do electronics, but haven’t soldered anything in 15+ years! I am comfortable opening the case and drilling holes as described in the documentation, but don’t know if my rusty soldering skills (or lack thereof) are up to the job…
3.
Is there a difference in where to tap into a camera trap’s circuitry depending on if you want to use the camera as a trigger vs triggering it externally?