Should declarative config create have a standardized mechanism for programmatic customization?

[jack-berg]

Potential followup to #4873 and #4777.

create now supports optional programmatic customization, but how that mechanism works is currently open ended.

Its currently framed as an escape hatch, but if use becomes common or important, it could be good to standardize the mechanism. One suggested solution is to incorporate references to the allowed hooks in the config data model, giving users more control over what happens.