Private unique device identifier in iOS

We're working on a project with my colleagues which involves using a lot of private and non official code. This is not intended for AppStore use.

The first and only requirement we have is to not use jailbreak.

First of all, UDID or OpenUDID or any other solutions don't work here and they're not expected to.

We've done a lot of background research and tests, starting with trying to get the IMEI, ICCID, IMSI and the Serial Number programatically. None of the above methods work with iOS 7 and above without jailbreak.

We've also spent a couple of months to play with IOKit framework using the famous IOKitBrowser and dumping the whole contents of iOS internals. Unfortunately, we discovered that with iOS 8.3 it stopped working.

We're talking here not about getting the UDID or any other "mainstream" thing, but generally speaking we need a way to get

any permanent hardware identifier unique enough to identify a device that would persist in spite of device wipes and amongst different iOS versions

This question is no duplicate to others (no solutions are found here, for example) and is targeting solely private APIs.

Any help would be highly appreciated.