Facility to limit country and state dropdowns per instance

The current Country / State dynamic dropdowns are great, and you have the option to limit the countries and states that appear in the global config. However, it would be far better if this could be done for each instance of the dropdowns as well.

My use case example:
We use a CiviProfile on the Drupal registration form to sign up new users as Contacts, however, we only want to accept new registration from either Ireland or the UK (actually, only a subset of the UK - Northern Ireland, but I'll leave that for another ticket). However, we are also using CiviContributions to take donations and want to be able to accept donations from any EU country. Currently, the only way to do this is to include all the EU countries in the global config, and then hack the select on the registration form in hook_civicrm_buildForm to alter the Country select, but it would surely be better if you could do this natively.