Extend standard page with a picklist that contains data from an external system

Question

The requirement is to extend the Campaign page with a custom picklist field that shows data (select options) stored in a 3rd party system (accessible trough a rest endpoint).

I know there is the possibility to include a custom visualforce page in the page layout. But there are some limitations like the inlined visualforce page can't be used in the edit mode of the standard page. This is a bit cumbersome and will also end up in an unfamiliar editing experience for the user.

What is the best/natural way to accomplish this?

Here are some (maybe strange) thoughts/hacks that i can image to get this working:

1. Only override the getter method for the picklist options value from the standard controller (combination of controller extension + apex:detail page)?
2. Add some custom java script that override the options in html?

This is the first time i use visualforce, so any recommendations and pointers to right direction are really welcome.

Answer 1

1. is not an option as far as I know. You can't override that

2. can be done, e.g. with some parts of this code https://developer.salesforce.com/forums?id=906F00000008kzBIAQ but then use your rest service as the data source. But it is seen as a hack, obviously, and requires you to include the Messages&Alerts on each page.

3. The solution prescribed by SF will be to create a VF page from scratch to override the standard Edit page. You will lose the standard page layout assignment, though.

Ideally, there should be an apex:detail component that allows edit mode, so you can put your javascript there, but unfortunately that's not supported (vote here: https://success.salesforce.com/ideaview?id=08730000000BrQtAAK)