When talking to benefits providers (dental insurance does not exist) they are pretty unanimous in that they pay what they pay and allow patients to upgrade as long as fully informed. This means a signed form (video is better) stating they are paying for a better restoration and know it is all out of pocket. There is no code, it does not get submitted, but for the ledger one can create a code in PM software. Codes not recognized by providers simply do not get paid. Codes created are often "cosmetic artistic upgrade" and such, usually used for complex cases or aesthetic challenges.
Dental plans simply don't want a routine case where "allowed" fee is $500 but the patient is charged $2000.
The key is informed consent, and it takes a dentist and staff that are willing to do the extra little things (signed form, manage ledger, etc). It is certainly more simple to just say "can't do it" and move on.