Arduino
Would be a latency free and simple solution, given the Arduino MIDI library. However, I am not sure you can receive from a device and send to another using USB-MIDI. Using a board with several Serial ports would work, but you need to convert your USB-MIDI signal to a "plain MIDI" with a DIN5 cable… And then the other way around…
Pi
You can run a very simple audio system on a Pi, using JackJack, and then use some utilities like QMidiRouteQMidiRoute, which is basically allowing you to change your (USB) MIDI signals any way you want. I would think it is much more easier than SuperCollider and so which are much more audio focused. If you can afford the booting time and need for it to be plugged, I think this is the simplest to do it! It could be able to route several guitars at the same time, like a transposing server!
Using a MIC
You can probably "lure" the mic by using a full wave rectifierrectifier. This will make a crunch signal at twice the frequency of the original. If I remember well, this is what is done by a lot of guitar pedals like Fuzz, Muff. If you have diods in it, you have a good chance they will rectify.
No definitive answer but hope it helps!