The recently published proposal for UI Hints in USD promised a further proposal would be coming to tackle specifically the encoding and behavior of the shownIf and displayGroupsShownIf hints. The propoposal, which covers the encoding as an expression language, the specific semantics, and how it will be incorporated into SdrShaderProperty, is now published as Conditional Visibility for UI Hints, ready for your feedback on the PR.
Thanks for your consideration!