i've recently came up with an idea how to flash locked/signed firmware on litebeam ac gen2. basically you start flashing same ubiquiti fw that is already on device and interrupt the process, that leaves mtd partitions unlocked and you can flash another image to these using dd. more info in this GH comment-> https://github.com/openwrt/openwrt/pull/689#issuecomment-493658317
let us know if it works on your litebeams/powerbeams too