Current Employer style field for a custom field
You might have already checked field like &quot;Current Employer&quot; that creates new organization and relationship. Post hook is the most straight-forward and correct way to proceed here. 

You could give a common prefix kind of name to your custom field and write your code in such a way that it performs the action only when custom field has that prefix in the title/name. This way, you won't need to modify the extension every time a new custom field is added.