the get parameters of this search is the array i said before, the search will return an array of [post_id , post_title], if found any, and will be used as select option.
You can also trigger the search when a minimum of words is typed, or just add some button.
test_initial_callback can be just the load of the field, if any value is found on database, for sure this is way better than loading an select with all possible options and populating the html.
I think you can do this and merge the feature on live (i think even something basic is appreciated by anyone)
I'd like to help you, but xdebug on wordpress/windows is not working, also i work mainly on Magento, and neved did something complicated on wordpress.
Personally I'd like to see this feature on post_relation plugin.