We made an integration via javascript that will populate dropdown based on API calls. We did this exactly to overcame the limitations of not having cascading dropdowns and not having id/value pairs in the database for dropdowns.
I am not sure if the code we got done is of use, and we are just finishing changing it to be able to call it from the external javascript functionality in the form options (we were doing by having an external php page that would call the form and inject the javascript - really messy but working for years now).
As soon as this is a bit more tidy, I'll see if the code can be shared and understood here.