This was not at all obvious to me from the Feeds API Section Reference or the code comments.
To get the status of several feeds in one GetFeedsSubmissionList call, pass an array of FeedSubmissionIds in the associative array you pass to the MarketplaceWebService_Model_GetFeedSubmissionListRequest constructor, e.g.:
$parameters = array (
‘Merchant’ => MERCHANT_ID,
‘FeedSubmissionIdList’ => array(
‘Id’ => array([FeedSubmissionIdList 1], … [FeedSubmissionIdList n]),
$request = new MarketplaceWebService_Model_GetFeedSubmissionListRequest($parameters);