> Several wifi-related bugs put the system in a state where it gets into a crash/reboot loop early on during boot/login.

Another perspective is: that's a risk of running dev channel, and there might be (rare) cases where the system isn't recoverable due to some sort of UI/driver/kernel crash bug.

There is understandable reluctance in adding keyboard shortcuts since they are (a) hard to discover, and (b) hard to understand (and undo) their side-effects when they get accidentally invoked.

Finding issues early on in canary and dev channels is the actual intended use of these channels.

Understand the concern around severe wifi issues since they prevent AUs that give us an easy way out to roll out fixes but I am not sure key combo is the right answer here. 

On developer builds only (i.e. not on released images), we could have the kernel poll the keyboard, and if the 'S' key is pressed (which I am not sure is detectable since the EC may sends keyboard state changes only), invoke upstart in a way that forces a "minimal" boot, ending in a root shell.  Then one could manually continue the init sequence with the proper "start" command.  I would have found this useful when debugging upstart scripts.  But it's never been proposed, probably because it adds some complexity, and folks can just change init scripts on their own.

In any case, if wifi is broken, it may be better for most people to do a recovery via USB stick than to find a USB-ethernet adapter and leave the laptop plugged in until the AU comes.