You are right, there are no functions at the moment to assign state values to properties in WAAPI, see
https://www.audiokinetic.com/qa/6291/how-to-get-state-groups-states-properties-object-with-waapi.
I'll take note of your request to help prioritize this feature request.
At the moment editing the XML is indeed the only way to edit those values outside of the Authoring application itself.
Note that for a plug-in, those values are expected to be set by the Sound Designer and can be copied over from other objects using the "Copy States Values..." button, located in the States tab.