What are the mechanisms involved when trying to shift content from private forks onto the public chain? e.g. if R3 did indeed fork Ethereum and they decide later to utilize the public chain, how difficult would it be on a technical level to do so?
As long as a fork of Ethereum remains compatible with the EVM (Ethereum Virtual Machine), all contracts on a private chain can be deployed to the public chain and behave identically: no code changes required.
If such contracts contain data, scripts can be written to store/migrate the data on the public chain.