I don't know in how far this is possible but here is an idea for implementing it with less lag:
Duplicate the Belt-Entity. One requires Power, one doesn't. Anytime you place Belts, Loaders or Power-transmition-stuff you check if at least one Belt is connected to the power network. If so, every Belt which is connected to the certain Powered Belt, will be replaced with the non-powered belt. For the consumtion you adjust the powered Belt.
Further optimization: Split the Belt network up into chunks and safe metadata per chunk (you may use the already existing chunks). One Metadata per BeltNetwork per Chunk could contain a flag: Powered/notPowered and the connection Points to the ChunkBorders.