Well, there's a little update...
Phi_prompt « Liudr's Blog
... You can guess where this is going. And yeah, it works 100% with the MPGuino as well - a 3-button mode is selectable (although a bit tough to navigate, I'll smooth out the wrinkles there somehow), and it's pretty much exactly what I had in mind. I'll have to tweak some things here and there, maybe, but I mean... just look at it! xD
Just define what buttons are what (the analog pins that the buttons use are digital 17, 18, and 19), and pass it a LiquidCrystal object, and it does the rest!