I also load Xorg into memory using libmlockall, which is the same as mlocker, basically. The solitary crash I had (yesterday morning) occurred also when the screen was off and the system idle. I have BFQ on both mmcblk0 and 1, and my swap is also on 1, so I am not convinced about it being swap at fault.

The next stage is to build the kernel without bfqio enabled, if you wouldn't mind testing it. I am unable to reproduce the instability in my own device, which is frustrating as it makes tracking down the cause difficult..