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The REC's could be purchased from Green-e list of sources, such as the Bonneville Environmental Foundation. They would be registered with an authority such as WREGIS to prevent double-counting and then tokenized on a public blockchain ledger. The utility bills would be placed on a permissioned Hyperledger Fabric utility emissions data channel. The REC's would follow standards such as those being developed by the Interwork Alliance.
The application utility bills would thus need to be able to interact with both public and permissioned ledgers. This would be done by holding keys to both ledgers in a common wallet, such as a Hardware Security Module (HSM) or Key Management Service (KMS)be placed on a permissioned Hyperledger Fabric utility emissions data channel.
The REC's and utility emissions would be calculated together on the net emissions channel.
An additional blockchain could be set up to manage members' subscriptions, so that new members could stake to join a network or purchase shares from exiting members. The exact mechanics would depend on the local regulations for community solar projects.