The idea of a phone cord is actually quite enticing. terminating one end is a simple matter of including a 4p4c jack on the MPGuino and terminating the other end should be doable with a generic wall-box (like
this one) and run what-ever type of wire from the wall-box to the Injectors, GND, Battery, VSS. Then a simple male-male phone cord can run through the firewall, in tact and sheathed. Should make for a very clean install with an easy to remove connector.