Auxilliary & Relic Commands

Come to think of it, MasterZed's Test patch (found here) does something similar to what you want. Hitting left (or L) brings up a new menu entirely instead of the Row command window, and the menu's contents depend on which of the four default commands you had highlighted before hitting left (or L).

Unfortunately, I don't think he included actual documentation, but what he did can probably be extrapolated with binary comparisons between the Test patch and a clean ROM. I know Hex Workshop is capable of doing such a comparison, but it's not exactly freeware.