Now that Gordon has reminded me of the SCI manual I would say you still have some hope. If a bumper sensor pulls the processor pin low when it detects an object you could wire photo-transistors (PTs) in parallel with two of the existing PTs. You have to pick which PTs you want. If either the upper or lower PT detects light then the MCU would see it as coming from one PT.
Then you have to figure out how to power the IR LEDs with the modulated signal. You would have to figure out how the LEDS are presently driven and then somehow add them to the circuit. I think you are going to need an o-scope to proceed with this project.
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