Won't work on iOS. An app cannot simply get the local MAC address on iOS. Privacy reasons. And trying all the (2^8)^3 options will also not work - for power reasons you'll be quickly throttled.
Also, 16777216 possibilities really aren't that many these days. With six cores at approximately 3.5GHz, assuming verification costs about 1000 instructions per key, brute-forcing every possibility will take between 4 and 5 seconds at most (half that on average). With appropriate rainbow tables, I think that should be feasible?
Using Core Bluetooth API it is trivial, but you need to either: a) create an app that does it and user has to download it b) modify SDKs existing in apps (e.g. Ad SDKs)
Also turning app/phone into a "BLE beacon" is only possible when app running in the foreground (on iOS).
Knowing the MAC makes the attack reasonable - let's say 5 hours compute for 3080Ti.
Not knowing the MAC makes it exponentially harder. You can still "guess" it, but the search-space is vast and that would take bazillion-years.
So to attack iOS device: - user has to download the app - app has to broadcast fake BLE - some other devices (e.g. Android/RasPi would need to pickup that MAC and pass it to you