TI currently promotes an capacitive input method with their MSP430 controllers, and there is a paper on that here http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slaa379&fileType=pdf
So, as an idea, just make a PCB with enough capacitive fields (at least 4x4 fields, more would be better), which could then be grouped together in software.
You could add a piece of transparent material into the kit, to cover the PCB and let the user place a piece of printed paper between that and the PCB, to label the sensitive fields.
Such a construction would accommodate those that just need a button or two as well as those folks that need a full number pad.
If there were enough capacitive fields , one could implement anything from one large button to sliders and wheels.
