Can monsters be made to display their status?

You are right on several points. It's certainly not the case that no one has thought of adding a method to indicate or check monster statuses. That's not to say it's impossible, but it is sufficiently hard to say the least. (Of course some are indicated already: imp, confuse/muddle/control, death/petrify, vanish)
The idea of adding the info to Scan as text is the easiest way, in my opinion, as text is fairly easy to work with and we have game examples to work from. 
Doing a tint or flash may be possible as well, given that monsters already flash when they attack, and there's that script command that tints red. 
I'd recommend not using the AI script to do it, as you noted that would lead to huge scripts. 
Something like a head graphic is probably the hardest. If you can just reuse the character ones, that's an idea. Mute, for instance. I dunno, it would be an awesome patch if done well.