Sorry for some delay with the answer and thank you for the request. We treat build status as immutable. To solve this and similar problems we plan to introduce searchable tags/annotations to many objects like build, build jobs, deployments, etc. Setting those tags with API will be possible as well. Cannot provide specific ETA now.
I am not sure I can provide you a workaround for now. Is that external test execution can be checked with some API from AppVeyor build?