Bitcoin by itself doesn't actually need much energy - it could easy run in a single server. The reason it uses so much now is because so many miners have joined, so the software increased the computational difficulty to keep generating 1 block every 10m.

If you run the bitcoin daemon in "regression test" mode, which creates a private, disconnected blockchain, it can actually generate ~200 blocks per minute using just a single CPU.

If a block chain is closed and the membership known, you could use some metric of "organizationally distinct signatories" to determine the length (/cost) of a chain rather than proof of work.