Change display of "vacancy" custom field to select

Currently the "vacancy" custom field displays as an integer - the id of the vacancy.
What we want is for it to display as a dropdown-select of vacancies.
CiviVolunteer faced a similar challenge and ended up doing some form hacks. I think a better solution would be to make use of http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_customFieldOptions
Core CIviCRM probably will need to be extended a little bit so that custom fields can be declared as type "select" with no associated option group (and rely entirely on the hook to provide options). This change will be very useful now and in the future, so IMO pursuing this would be a better use of our time than going the form-hack route again.