Assuming the asset is not locked yet, is it possible to implement or support CIP-0026 on a previously minted NFT without royalties?
It is not possible to enforce royalties on previously minted nft’s. There are no restrictions on how an asset is send if it resides in a “normal” well behaved address. That is, the current owner can send it without any logic other then his own.
However, these assets can be brought back into such a royalty system if the current owner agrees with that. What one can do tho is create a multi-sig address that needs two signatures to spend an utxo. The first one is that of an owner. The second one is that of a plutus script that enforces two things.
- In the transaction royalties are payed.
- The asset is sent to a similar multi-sig address that abides these rules.
But again, note that for previously minted nft’s to abide to this system they have to be sent to such a script address in the first place, and the current owner has to decide whether they want to do that. In addition to this owners have to have a service to index which nft’s they hold since normal wallets will not index such multi-sig addresses. There is a CIP about this 
What is also being done is that market places integrate these functions. That is, they make plutus scripts that enforce the royalties (see the plutus script of spacebudz as an example ). But an owner of a previously minted NFT can always go to another marketplace that does not have such a function to optimize the trade and maximize their profits.
In the meta standardization is there something for this? Wherein you would input say the creator’s cardano address (or an array of addresses of contributors)…and when we are developing apps that interact in this way, we can look for that field in the meta and automatically implement rewards payouts? Then marketplaces would have an additional healthy competition layer: their royalty % they decide on…so in other words keeping it simple and have the market decide on royalties rather than a complex royalty meta structure…when ultimately any NFT could be resold without constraining to a marketplace that has royalties at all…that would be my thinking as to doing it with markets deciding vs artist/creator. Which will then speak to marketplaces priorities, etc, further giving layers of differentiation that are pretty healthy for the ecosystem imo.