Thanks to new info from Johannes, I was able to finish setting up the modbus IOC on c1susaux2. It turns out the 17 Acromags draw ~1.9 A, which is way more than I had expected. Hence the reason I had suspected a short. Adding a second DC supply in parallel solves the problem. There is no issue with the wiring.
With the Acromags powered on, I carried out the following:
The modbusIOC is now running as a peristent system service, which is automatically launched on boot and relaunched after a crash. I'm able to access a random selection of channels using caget.
What's left now is to finish the Acromag-to-feedthrough wiring, then test/calibrate each channel.