Contribution On Behalf Of - alternate relationships

Per http://drupal.stackexchange.com/questions/136661/civicrm-membership-contribution-with-profile-required-on-behalf-of-prefille/136711

The "On Behalf Of" functionality hard codes an employer/employee relationship for the contribution page – but there are other custom permissioned relationships that it would be useful to have the pre-filled contact information for the organization.

The help text here already indicates that "logged in users will be given a choice of selecting from a list of organizations that they have a permissioned relationship with OR entering a new organization" but that functionality doesn't work yet.

I have a rough start here:
https://github.com/laryn/civicrm-core/compare/patch-1

But it's not perfect yet. It works correctly if the contact has two permissioned relationships (generating the select dropdown) but for some reason if there is just one permissioned relationship, the name is not pre-filling in the field.