I bought ASUS ac57u for usage with OpenWRT, but got V2, which is QCN5502 not MediaTek. Bricked it, unbricked with stock (soldered 3 pin serial interface, it was my first soldering but it works).
Anyway, I spent on it already way more time than I could. Stock version is useless to me, but I would really like to see OpenWRT to support this device.
Therefore I offer to donate it to a capable developer dedicated to port OpenWRT to it.
As I said the serial interface is soldered, and I even let the cables out, so one literally doesn't need to open it to get to uboot console. I think I did not break anything, stock image works, it's just useless in my configuration (I wanted basic DNS inside network and to connect with fiber network).
Please contact me if you're interested. My expectations are that if you fail to succeed you will pass it further to someone else. If you succeed you can keep the router as long as you publish your instructions online.