A client I work for bought a job lot (several hundred)  AirMobi iPlay2 units.

These were quite popular until recently, and are based on the MediaTek MT7620A and a Wolfson WM8960G
They are basically Wifi DLNA receivers.

I dumped the SPI Flash and it looks to me like they run a build of busybox.

I'd like to be able to modify the firmware on these.
I can re-prgram the SPI flash chips directly, but I'm not sure how to build the image.

Can anyone offer any advise on how to build the image for these MT7620A devices?