What would be nice might be to be able to define the prefix for /etc to something else (so we can use atomic symlink changes to flip configs).

 Make /etc a symlink to /var/config/etc. Make /var/config a symlink to config-0.
Flip configs by atomically replacing the /var/config symlinks.
 You'll have to move /etc, or part of it, out of the root filesystem anyway, because
you'll want to make your / read-only: so, kill two birds with one stone.

PS As an aside, I've always wanted /etc/hosts to also have a parallel /etc/hosts.d/.
 It'd make maintaining things without a DNS server extremely easy - think dynamically
 adding and removing VMs in most cloud providers, especially those where multicast DNS
 doesn't work... like Azure.

 Honestly, using djbdns is almost as easy, thanks to the programmatically sane config
file formats; and it will scale better.

--
 Laurent