As I understand the situation, no.
Since the AirTag emits the message, that message would either contain:
- a static signature, which could then be copied and mimic'd by imposters (replay attack)
- store the private key on the AirTag device, which could then sign the a continuously changing nonce like the current datetime. But this means the private key could then be extracted from one device, and used to sign messages on an imposter device. So unless every device had a separate private key, this method would immediately be compromised as well.
So why doesn't Apple have a unique private key for each device? Well it appears it actually does, and has them constantly changing their private keys. But there appears to be some kind purposely implemented anonymity features that is designed to prevent Apple's servers from associating a ping with ever having to decode the contents, and thus of associating your account/device with the emitted location.
If you build in validation to write to the network ping database that goes "here's a ping with a signature ABC and let's lookup if it's valid, oh it is, that must be from AirTag Bob bought last month with private key XYZ, let's declare this ping valid" then Apple is only a logfile-write-of-this-information away from being able to perfectly stalk everyone who has purchased a device. So instead, the tradeoff they made is they don't keep track / purposely blind themselves to their device-in-circulation keys to truthfully say they actually can't track you. That leaves open the ability of imposter devices to transmit information through the network by creating their own known keys which look indistinguishable from authentic device pings.