Answering my own post here, just to add info:
To get DevMan to recognise the UPS and add required info into DevManBE.ini, it needs to find the UPS once, so you either connect it to an available port, then run Setup, then edit the ini-file, or easier:
1. Remove the existing ttyS7: rm /dev/ttyS7
2. Link ttyUSB0 to ttyS7: sudo -ln -s /dev/ttyUSB0 /dev/ttyS7
3. Install, selecting ttyS7
4. Edit /usr/local/DevMan/DevManBE.ini:
[Comm]
Port=/dev/ttyUSB0
CC=0
5. Restart DevMan
Note that the port link will not survive a reboot, so it is a temporary measure, just to get DevMan installed.