How to stop .dmgs from automatically mounting

I found this forum post which has a couple options:

The straightforward way is to create a login applescript, replacing /Volumes/Windows with your path:

tell application "Finder"
    if (exists the disk "Windows XP") then
        do shell script ("hdiutil unmount /Volumes/Windows\\ XP -force")
    end if
end tell

Another way is to modify /private/etc/rc.local to add:

disktool -u disk0s3

If you have more than one disk, or have formatted it into more than just a MacOS and Windows volume, you will have to use Disk Utility or something to determine the Disk Number (in this case 0) and Partition Number (in this case 3) of your Windows volume.

After a lot of searching and much cursing, I think that this is really a problem that practically nobody is aware of or concerned about. Still, I was able to find a solution, it's not great, but it should work. The idea is to mount the DMG in a known directory with some random name -- this name (the last element in the -mountpoint path) will be the name property of the disk. So using this I walk over all of the disks and look for ones with a matching name -- complain if I find none or more than one, otherwise, I have the needed object ready to work with.

mnt="some-random-name-$$"
hdiutil attach -readwrite -noverify -noautoopen \
               -mountpoint "/tmp/$mnt" "/tmp/out.dmg"
/usr/bin/osascript <<-EOF
      tell application "Finder"
        set myDisks to every disk of desktop
        set theDMGDisk to ""
        repeat with d in myDisks
          if name of d = "$mnt"
            if theDMGDisk = ""
              set theDMGDisk to d
            else
              error "Too many attached DMGs found!"
            end if
          end if
        end repeat
        if theDMGDisk = "" then error "Attached DMG not found!"
        -- found a single matching disk, continue
        tell theDMGDisk
... etc ...