That switch is described as a "non-shortening" one which basically means it is a "break-before-make" (B-B-M) This would result in total loss of value as the switch moves between stations. With 12 positions (and no "dead" positions) there will be 30 degrees of rotation per location. In all probability a B-B-M arrangement will have a 50% contact area so at best you will experience 15 degrees of known location followed by 15 degrees of no signal etc etc. If you want to use a 12-way switch as a rotary encoder I'd suggest a "make-before-break" (M-B-B) would be a better option. This will then give you 30 degrees of known location followed by 30 degrees of the next location etc etc. Because of the M-B-B feature there will be several degrees of "error" until the break has taken place. However as this will be similar for all locations it simply results in an off-set from the indent positions which can be accommodated by mechanical offset.
This kind of part, with 1 common and 12 contacts?http://www.parts-express.com/pe/showdetl.cfm?partnumber=023-660I could see wiring it up as voltage divider, read the common on an analog input, map each reading to a 1-12 number.
Do you mean you want your 12 way switch to produce a 4-bit binary code4 bits equate to 16 options so if you could use a 16 way device here is a commercially available product that doesn't require any intelligent processor http://www.lorlin.co.uk/PDF/BCK.pdf