As a DIY project I’d agree, but if you’re doing manufacture it’d be far simpler to have it all enclosed in one system. A simple turbine just prior to the outlet in the cistern could recharge a sealed battery moulded inside the cistern, with the wires running through the seat hinges.
Multi-colour LEDs controlled by a dial near the flush handle could allow a change of mood, from aqua calm, to red frenzy.
