Jon,
have you seen nmcli? If you can't do what you want to with it, maybe a
feature request is in order.
I get the impression that NM will generally do the right thing for
most people most of the time, and when we are hacking on firmware,
running a tftp server and the like we might want to use nmcli to take
a device temporarily from NM's clutches and take manual control. I'm
not sure it that's possible though with the current nmcli.
I tried
nmcli dev disconnect iface eth0
on my netbook which was connected to wlan0 and had nothing on eth0 -
it errored saying that eth0 wasn't active to start with.
I think keeping NM running for the system is a good thing, taking one
device over for your own purposes is a better thing to do *if
possible* than disabling NM entirely.
-Cam