I don't know about the icon, but yes, that's the general idea I'm thinking of.
In regards to what should go there - perhaps one possibility is even to make this user-configurable (I thought of this while I was going to write up a point about the frameskip buttons, thinking "Why do they need to adjust based on the hotkey config?" which lead to "Well, they should be configurable, but why not just make them configurable independantly?" and then finally to "Why not just make the extra buttons configurable altogether?"). We'd still need to choose defaults, though.
Perhaps this could even be expanded to including the Pause and Nuke buttons in what's configurable. We have room for 20 buttons. Take away 11 for the 8 skills, the 2 release rate buttons, and the show/hide extras button. This leaves room for 9 buttons - it could simply be set up so that the user can pick these 9 buttons, with the first two displaying at all times and the other 7 only when the minimap is hidden. Perhaps even "Show Hotkeys" (or even simply "Edit Hotkeys" - as far as I can tell, the code *should* be able to support live-changing of them during play) could be hardcoded to be the last button rather than configurable, so the user can never truly lock themself out of being able to do something.