Another idea I just had. Rather than use webhooks, which would require some public HTTP endpoint to be registered for each test runner, we could continue with the polling mechanism by:
make.wordpress.org/hosting publishes an endpoint with the latest revision.
Each test runner instance polls the latest revision endpoint. When the value changes from the local value, the test runner proceeds to run the tests.
It should be possible for the test reporter to trigger Travis jobs against each environment when a new changeset is reported.
WordPress.org SVN webhook event -> test reporter -> DreamHost, WPE, Pantheon, etc. runner repos.
Ideally, the test reporter would create mock result posts for each runner repo so you can see all of the environments about to be run.