I haven't looked under the hood to see how this works, but I'd wager a guess that the code that does this is written in Objective-C. Thus, it would be unable to access anything from Swift that wasn't exposed to Objective-C though @objc.
To my understanding, lldb does not at present look at CustomPlaygroundQuickLookable conformances. So from a "maintaining parity with current behavior" perspective -- which is the goal of this proposal -- confining the new CustomPlaygroundDisplayConvertible to something playground-specific is correct.
Similar to a sentiment expressed in a previous message, I also think that there's room for CustomPlaygroundDisplayConvertible to live alongside CustomDebugDisplayConvertible should that someday be added to the language. Playgrounds and the debugger are different environments, so being able to customize display for one vs. the other makes sense to me. (I imagine it would be likely that both environments would check the other's protocol, but at a lower priority.)