What you call the "antenna processor" is the
baseband processor. There wouldn't need to be a separate anti-theft chip. What you describe is well within the capabilities of the baseband processor itself. It's a
small matter of firmware.
In fact, every phone already has the fundamental capability for this feature: it's called the
IMEI number. It's unique to each device (specifically, to each baseband processor chip), it's difficult to change, and it's already communicated to cell towers and cell service providers. From the afore-linked Wikipedia article:
The IMEI number is used by a GSM network to identify valid devices and therefore can be used for stopping a stolen phone from accessing that network. For example, if a mobile phone is stolen, the owner can call his or her network provider and instruct them to "blacklist" the phone using its IMEI number. This renders the phone useless on that network and sometimes other networks too, whether or not the phone's SIM is changed.
The main addition would be to the Application Processor (AP), i.e. the A6 or main SoC. If the baseband processor has firmware that detects the disabling of the IMEI number when told by a suitable message from the cell network, then the AP can ask the BBP if it's disabled, and then perform the remote wipe.
Or for the Find My Phone feature, the BBP receives the message from the cell network that a Find request has been made. The AP sees this and provides any location details it might have, otherwise the BBP by itself can provide a
cell-tower triangulation location with no assistance from the AP.
None of this needs an external case, but it does need capabilities that can only be provided by the cell network provider, and by the provider of baseband processor firmware. It's a fairly natural extension of disabling an IMEI number, by making the device itself (i.e. the phone) aware of the IMEI disablement and having it act on that information.
None of this needs an external smart-case, either. It's all done in software or firmware using the chips already present in any phone.