using specific parts of objects as buttons?

I have a computer monitor in my game and want to use onscreen buttons. At the moment the only way I can think of is to have another object in front of the screen with the same texture and have the button script attached to each of these buttons.
What I want is to be able to click directly on the monitor and tell what I clicked on. In the end I want the screen to be able to change or scroll and have the buttons still work.
My idea was using a visible texture map for what the player sees and a secondary one that the engine could check if the mouse is over, that way I could use some sort of tracing function to check what color I am over and execute the appropriate button function. It would be nice to be able to change the hover and click texture too (I can do that no problem with the separate object method, but can’t scroll the screen or have it just flip to a new screen. Plus it means I have to model each button and it’s UV coordinates)

here is what I was thinking, but would need some help getting it running.

3503-sample22a.jpg

as you can see one is just normal buttons, the other has a separate color layout for each one so that leaves a possible 16+ million buttons of any shape. (even using a single alpha map on a non transparent material would still work if it could detect what gray level is used for 256 possible buttons (not that I plan on using anywhere near that many).
Please let me know if you can give me some idea how to do this
thanks.

So basically you want to do something like this: