The dispatcher does append Action or Method like this for any named method, but your limitation here is the Router. This is a bit of a limitation because of Alloy's REST preference (a Controller is a REST resource), and as a result the controller will only have one method each that can receive a POST, PUT, and DELETE request.
Currently, you would have to specify an action to route to for POST requests:
$router->route('test', '/<:identity>(.<:format>)')
->defaults(Array('module' => 'Test', 'action' => 'index', 'format' => 'html'))
->post(array('action' => 'index'));
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Vance Lucas
http://vancelucas.com