Create a MIDI matrix grid page, where the inputs and channels are listed along the top and the outputs and channels are shown along the right hand side, then use the pads to select routings.
Allow all inputs and outputs to be shown in one summary page (even if there are multiple devices and channels on one routing, but make it clear that's the case. Allow filtering by input and/or output device. Also allow routings to be filtered by channel - achieve this by holding the page and then selecting the channels to be filtered out.
Where would it go?
On its own MIDI page.
Any details or mockups?
Taking inspiration from AUM and Apematrix on the iPad, both of which have great MIDI grid systems.