Linux Filesystem Tree

I think I found the answer: blkid

From the man page:

The blkid program is the command-line interface to working with the libblkid(3) library. It can determine the type of content (e.g. filesystem or swap) that a block device holds, and also attributes (tokens, NAME=value pairs) from the content metadata (e.g. LABEL or UUID fields).

Apparently it prints the device name along with the filesystem type (along with some other useful information). To get a list of all devices with their types:

blkid | sed 's!\(.*\):.*TYPE="\(.*\)".*!\1: \2!'

To find all /dev/sd*/ devices, just add in a grep:

blkid | grep "/dev/sd.*" | sed 's!\(.*\):.*TYPE="\(.*\)".*!\1: \2!'

Then just cut or awk to get what you need.

You can use a symlink, I just tried this to be sure. Since the stuff in /tmp is impermanent, that means you will have to create the file at boot before dhcpcd runs.

touch /tmp/dhcpcd.resolv.conf
ln -s /tmp/dhcpcd.resolv.conf /etc/resolv.conf

I included the ln bit by way of illustration but note that you don't have to create the symlink every time at boot; just put in your RO system once, and (again) make sure the /tmp file it links to is created before anything tries to access it.

Symlinks share their permissions with the file they link to, and even if the filesystem they are on is RO, as long as the file they link to is not, writing will work.