In what way does ⌘+ (Mac) or Ctrl+ (Windows) not satisfy your need? I prefer changing font size that way, because it's simple and works on every site, and I can change it depending on whether I'm using a big monitor or not.
Is it possible? Of course... if you can change the topbar color there's no reason you shouldn't be able to change the font size. As with the topbar color, changing the font size wouldn't even have to affect anyone but the current logged in user.<p>Will it happen? Probably not.