From PR https://github.com/ManageIQ/manageiq/pull/12602: Create service_model mixin for exposing custom_attribute methods. Updated existing models to use mixin. Added mixin to ext_management_system service model to support functionality added in PR #10503 http://talk.manageiq.org/t/acess-drb-remote-object/1869
Verified in 5.8.0.3. I was able to use custom_(keys|set|get) on :ExtManagementSystem via $evm.