I can confirm the J2 is connected to the reset button, so pressing reset button does the same thing.
I managed to boot into Broadcom design reference firmware and the MR32 is quite usable as an AP in this mode, it broadcast SSID, support WPA2, etc ... the problem now is how to make the MR32 always boot into this diagnostics mode, it really tried everything and cannot make it persistent.
I know it's not running OpenWRT but at least it's usable at some extend.
To get into this mode you have to power on the device with the reset button pressed, when you see the white led flashing release the button and press it again, you will see the blue led activated, this means it's in recovery mode. There are two options, RED led means Meraki fw, BLUE led means Broadcom diagnostics firmware.
Now, the question is how to make it always boot into this Broadcom mode ? If I really don't find a "software" way to do it I will simply put a small Atmel Attiny85 controller inside to press the reset button sequence every time it boots...