There may be an issue in the library, or this may be that the code I wrote is using it incorrectly.

si = SignedIdentifier()
si.id = 'testid'
si.access_policy.star = '2011-10-11'
si.access_policy.expiry = '2011-10-12'
si.access_policy.permission = 'r'
identifiers = SignedIdentifiers()
identifiers.signed_identifiers.append(si)
self.bc.set_container_acl(self.container_name, identifiers)

This results in:

WindowsAzureError: Unknown error (XML specified is not syntactically valid.)