We should return a stub only in mw.wikibase.getEntityObject and defer methods like getLabel to PHP. Only if the raw data is being accessed we would load the actual entity into Lua (that logic would be in a __index metamethod).
Things to consider/ check:
- Does this break backwards compatibility in some way?
- Are these tables inmutable then? (They probably should, but that's another issue)
- Are there any unwanted performance implications of __index we need to be aware?