The part I don't understand is why you want to install it in system space in the first place.
We never have. Install it somewhere you have write access to and add that to your $PATH?
Support modules should be shared, so they need to end up in a global place with EPICS Base and the other shared support modules.
And, /opt is actually not strictly system space. It's intended for optional shared software modules. So, for something like EPICS (Base and support modules), a place like /opt/epics would be a canonical location that is perfectly in line with the applicable filesystem hierarchy standard [1]. (The only missing thing would be the registration of "epics" with LANANA as a "provider" under /opt.)
Cheers,
~Ralph