The Amazon S3 url is hard-coded with the "http:" scheme:

const S3_ENDPOINT = '<a rel="nofollow" href="http://s3.amazonaws.com">http://s3.amazonaws.com</a>';

Line 356 of the method "_makeRequest" sets the uri of the Zend_Http_Client object as a string:

$client->setUri(self::S3_ENDPOINT.'/'.$path);

The setUri method of Zend_Http_Client can accept a string or a Zend_Uri_Http object. If instead of a string, the uri above were an instance of Zend_Uri_Http, then the uri and scheme (http or https) could be set in Zend_Uri_Http's constructor. So Zend_Service_Amazon_S3 could allow a setter for protocol scheme and/or constructor flag.

This is all assuming that there are no other barriers to using Zend_Http_Client and ssl with Amazon S3.