Hi Marc,
Are the SQL queries modifying the input typed in by the user? Or are they just to pre-fill a list to choose from? They should be the later..
The SQL in the GUI should only be used to make it easier to enter values, for instance by searching based on other fields.
For Web Services, I would recommend creating Filters/Finders for the selections you want to make. These can be called directly.
Each field would require its own Filter/Finder, and this is how you can handle inter-dependencies between fields.
Retrieving the workflow base object via web services will evaulate each UI SQL, but because there is no interactive input here, the dependancies cannot be evaulated. ie. If the pick list keys Volume by Array, then Arrays will be listed but Volume will be blank because the Array name will be empty string.
Hope that helps explain it. It short, Filters with Finders returning the values you want are the way to go to provide pick-lists to up-stream portals. You can set the sorting order if you need and other options with finders.
Note: Many of these filters and finders you need are probably already there. For rest calls take a look at <wfa>/rest/filters or <wfa>/rest/finders, and give the /test method a go.
Kind Regards,
Michael.