It would be better to have a beacon that simply broadcasts that you do not want to be tracked, with no further identifying features. There isn't really a good reason for identifying you to then look up that you don't want to be tracked. Make that legally binding and enforce it.
How would opt-in work in practice? Say, if this gets pushed out on $CAFE public wifi for analytics. Would it be something akin to "tick this consent box to use the wifi"?
And if $CAFE tracks you regardless of you not ticking the box or connecting to the network, how do I detect that as a regular customer?
Or, better yet, make it totally opt in.