Vendasta lets partners create and manage custom object types and their fields through its external API. A field defined once exists in every account, replacing edits made account by account.
In each business's account, the partner's fields are read-only and the object type can't be reconfigured. Businesses can still add and edit their own fields on top.
Custom object types with field extensions can also be added to blueprints as components. Partners set up a partner service account for the external API, then follow the "CRM" section of the developer portal for the field extensions guide.