"the 2900s ran VME/B (and wasn't there a smaller VME/K?)"
Yes it would indeed be one of the Virtual Machine Environments that would be the appropriate OS for them.
"the "orange Leos" which were ICL 2900s running the 1900 emulator running the System-4 emulator running the LEO emulator running LEO programs for which they no longer had source code."
OMG It's been ages since I saw an emulation stack like that. I thought they only existed in the financial sector.