I think I found my answer. I would love confirmation though. Maybe I should take this to forums instead of answers?
I found this document promoting AVB recording coming to RM mixers in 2015 (pages 6-8), just what I want to do. Then I found this release from PreSonus saying it's not possible due to a chipset limitation on the Ai platform. So I think my answer is that the computer must be connected via firewire for Studio One. Solutions would be (1) a KVM over ethernet to give keyboard, mouse, monitor to FoH; (2) a second PC at FoH and screen sharing software; (3) a super long firewire cable, probably a bad idea because max cable length in firewire spec is ~15 feet; (4) another RM at FoH just for the firewire port; or (5) a Dante card for the RM unit, but it's very expensive and renders the audio i/o on the CS18Ai unusable.