I've pushed a compiling implementation of the DSL and the API we discussed.
What is missing:

• review the TODOs in the code and make a decision
• handle exceptions when the field name does not exist and if sortfield.type is detected
• decide if we want to let the DSL accept the type explicitly via something like onField("foo")withType(FieldSettingsDescriptor.TYPE,NumericFieldSettingsDescriptor.NumericEncodingType) (note quite sure what to pass otherwise to make it non Lucene dependent).
• convert my quick new SearchException into a proper exception (maybe also AssertionFailure)
• write some tests