You seem to be after a solution that's the least possible effort for the developer, whereas I'm after one that's the least possible effort for the user.
The more clicks, reading, thinking that a user has to do to work out how a feature is used, the less likely they are to work it out. A button on the page that performs a function requires a lot less thought on behalf of the user than a button that opens a window that contains some text which explains (in terms they might not understand) a function that requires a combination of 2 keys pressed simultaneously to perform.
Let me illustrate, with a few examples of what the text might say:
"To change text size, hold the control key and scroll the mouse wheel"
- what's the control key? The one labeled Ctrl? How was I supposed to know that?
- what's the mouse wheel? Is it the grey thing on top of the mouse, or the ball on the bottom?
- I held the key, then released it and scrolled the mouse wheel. It didn't work.
So we could improve it by saying:
"To change text size, hold the control key (the one labeled Ctrl), keep it held, and scroll the mouse wheel on top of your mouse (between the two buttons)."
- There's 2 keys labeled Ctrl - which one do I press?
- There's no wheel between my mouse buttons, I use a trackball/graphics tablet/trackpad/switch/voice commands
As you can see, in order to produce text with the desired effect that will ensure EVERY user reading it knows exactly what to do first off, it easily becomes quite verbose. The longer it is, the less likely it'll be read/understood. Plus as it's aimed at visually-impaired users, the font has to be quite big. Plus if you have to click a button to get to it in the first place, that button might as well perform the function you're teaching the user about, mightn't it?
Anyway, it seems several people here disagree with my approach. That's absolutely fine. I'm not suggesting it becomes a core part of Moodle. If you find that telling your users how to use the browser's built in zoom feature is the best solution, please continue to do so. For the rest of you, my block's free for you to use as you see fit