Contact edit form relate field filtering

I would like to filter the Contact edit form account field to only allow a parent account to be selected (i.e. one that has the account parent_id to Null or “”). I have been using the initial_filter for other fields but can not work out if it is possible to use it in this situation, I’m guessing I need to add/modify code rather than use the initial_filter feature?

Also, initial_filter appears to only filter the popup window, it doesn’t appear to have any effect on the autocomplete feature when typing directly into the field. Is it possible to restrict the options the autocomplete returns, or must the selections be checked once the Save button is pressed before the changes are saved to the database?

This is using suitcrm 7.21.

Any ideas / suggestions would be great.