@DrAS - From a property related API call, you can retrieve all possible options for a dropdown list (as you have found) as an array. Applying this array (or subsets of the array) to a search API filter can be done with an IN filter type and values array.
Note these API calls can’t be made from client side JS (only HubDB APIs are permitted in this context) - so it seems likely that the HubL option offered in this thread is going to be the best option for the display you want.
Good luck!
Steve