FreeForm fields: Need access to customAttributes
A suggestion from Freeform support:


  The only way you might be able to achieve this would be if you used a regular Text field (inside of Composer), and then override the formatting template to manually render the field as a select dropdown and populate the options dynamically. Actual fields with options such as Select fields need to have all of their options pre-defined inside Composer, which is why it wouldn't work otherwise. Using the element populators could almost get you there, but it doesn't allow dynamic filtering like what you'd need.