Answer: not very secure - but probably enough to deter casual unwanted connections, since it is technically possible to fake MAC addresses.

As for your original duplicate IP address problem, MAC addresses offer a solution if your DHCP server has a "Reserved IP Address" feature. The idea is to give all your Pronto panels dynamic IP addresses but register their indiviual MAC addresses with the desired IP addresses in a table held in the DHCP server. In this way each panel always connects to its own IP address that remains fixed.

The advantage of this arrangement is that you ony need to maintain one .XCF file that can be loaded without IP address editing into many panels, yet retain the benefits of having a fixed and known address.