Hi Martin,
I do not know how you want to access this, but basically it sounds like you want to have a filtered select list. With the help of the platform (REST) it would work like this:
Make a REST call for the select list to get the whole select list
https://[server]/DocuWare/Platform/FileCabinets/[File Cabinet GUID]/Query/SelectListExpression?dialogId=[Dialog GUID]&fieldName=CONTACT
To filter the select list you have to add some body (to the REST call) with the query condition e.g.
{"ValuePrefix":"","Limit":100,"Typed":true,"ExcludeExternal":true,"DialogExpression":{"Operation":"And","Condition":[{"DBName":"DOCUMENTTYPE","Value":["\"Photo\""]}]}}
I hope this makes it clearer.