I disagree that the proposed design is unnecessary boilerplate. It is an approach that is consistent with the rest of Swift whereas your suggestion looks like ad-hoc magic.
Further, global actor declarations will be extremely rare relative to global actor usage. Your approach increases boilerplate at the usage site which will result in significantly more total "boilerplate" than the proposed design.
Unrelated, I noticed that the text in the proposal just mentions a static property shared while the GlobalActor protocol goes further and specifies: "The value of this property must always evaluate to the same actor instance."
Normal Swift protocols cannot express a constant requirement, but given that this is a magic ad-hoc protocol in the compiler I think it would be a good idea to require shared to be a constant. Are there any valid use cases where it would not be @Douglas_Gregor?