without the need of a separate 13pin guitar pickup and a 13pin to 5 pin midi converter)
If it were possible then why do you think the commercial guys go to the bother of a 13 13pin guitar pickup?
There is great pressure to make things low cost and commercial developers don't make a complex system just for the hell of it.
Suggestions here sound fine but will not work, it is a much harder thing than you imagine. Just try it and see.
Switch mode band pass filters take too long to produce an output from a valid input and scanning a whole lot of notes is just too time consuming. Especially when you consider the post filter peak detector to decide which is the biggest. Optically you have the same problem as electrically, there is an oscillation and you have to detect the frequency. The problem is that most of the time the software gets it right but not enough of the time to be reliable and usable.
Hey I am called Grumpy after all and would love to be proved wrong. >:(