Merlin since you chat with ASUS directly see if there a way they could make a test bios that I could put on my 68u that talks to the log on my 66u. I have the 66u in repeater mode if it was possible it could send diag info to it then the reboot issue may get sorted out quicker. I reboot at the least once a day.
That is precisely what they are looking into - trying to devise a way to collect more debugging information. That's not easy, since the router flats out reboot, and that info is usually only sent over the serial console. This is how the Linux kernel works, not something specific to their FW.