According to current technical standards, the Rasperry 5 together with suitable very low latency audio and midi interfaces like PiSound (from Blokas) and free guitar effects software like MODEP (Guitrarix) could then be operated for about 4-5 hours with 6 lithium batteries of 3500 mWh.
And so you could build a foot pedal that could possibly even communicate wirelessly with the Android phone via Bluetooth.
And if the Raspberry or the Android phone also streams audio directly to an audio amplifier via Bluetooth, we would have a perfect little “wireless MG3” 
Possibly the easiest first project could be in MG3 vanilla…