How hard would it be to add a signal connection to the roboport part of the substation? Much like you can connect a wire to a roboport.

I figured it out (I don't like hacking other people's work). I used the chest connector with your mod, and it places the wire connection box in a decent place. Just add this into data.lua:

circuit_wire_connection_point = circuit_connector_definitions["chest"].points,
circuit_connector_sprites = circuit_connector_definitions["chest"].sprites,
circuit_wire_max_distance = default_circuit_wire_max_distance,

Then, the roboport portion of the substation will read out the contents of the logistics network. Cheers!