Create validation plugin inside core which a datasource can be used. One popular use case for this is verifying uniqueness of entities across a specific data set like users. Perhaps the default behavior is when results are returned the field is invalid. When the result/response is empty the field valid. That seems like a simple way to handle things. The validator can have a param that references an existing datasource inside the same panel just like with the form element to populate options.
Create validation plugin inside core which a datasource can be used. One popular use case for this is verifying uniqueness of entities across a specific data set like users. Perhaps the default behavior is when results are returned the field is invalid. When the result/response is empty the field valid. That seems like a simple way to handle things. The validator can have a param that references an existing datasource inside the same panel just like with the form element to populate options.