If you use any hardware that comes with macroing software, you can do this pretty easily. I'm sure autohotkey would work as well, which mutiboxers use all the time. As long as you don't do any serious botting with it, I doubt you'll get in trouble.
@ (target=) only works with UnitIDs, and I'm not sure if banners count. It's usually just people in your party/raid or specific frames (focus, target, targettarget, etc). You would probably need to do something like this:
Code:
/tar Demoralizing Banner
/cast Safeguard
/targetlasttarget
The full macro could look something like this:
Code:
/cleartarget
/tar Demoralizing Banner
/cast [mod] Demoralizing Banner; Intervene
/targetlasttarget
Holding a mod will give you the demo banner, no mod will Intervene to it. It maintains your current target. You should set the simulated click to go off in addition to the hotkey+mod itself if you press the hotkey when a mod of your choice is held down. You could go one step further and completely automate it, but I'm not going to detail that here. Pretty easy stuff.
It should work pretty well by itself, but you can figure out the specifics of the simulated click part on your own pretty easily.
---------- Post added 2012-10-12 at 01:01 PM ----------
No. Macros can't check CDs. If they are not on GCD, you can game it a little bit, though.