r/o root and /etc/udev/rules.d/*persistent*rules

> If you want to do that to mount it read-only, then you are simply aiming
> too low, and you should rather work on making / read-only.
(diggin' within /lib/udev) BTW it's interesting how you're going to do with persistent rules which are essentially the cache (thus belonging to /var, not /etc) but may be crucial to bringing up the stuff to mount said /var, be it a network interface or FC-connected storage (a tape library over here used to perform a streamer dance until fixated that way).

Not a nitpick, it's quite interesting, actually.