Restrict access to specific mobile devices (via IMEI number?)

You can execute authentication on any logic based on the information you receive from the client. But if the client doesn't send anything to uniquely identify itself, then there's nothing you can do. Whoever wrote that original article obviously had a way to get the client to send the imei which they did not post there and quite possibly no longer works (just as apple has restricted app access to uuid). You would have to determine how to get your clients to send unique identifying information (for example via stackoverflow or apple) and then you could use that irule or similar one to extract it.