Here is a spell for retrieving MARCXML records from the Library of Congress's SRU server. It relies on the fact that IndexData-written SRU servers (such as Metaproxy) offer a convenient way to pass Z39.50 PQF query strings, so we're not stuck using just the predefined SRU search attributes.
Here's an example of retrieving all NAF records that were last updated on 2016-05-16:
http://lx2.loc.gov:210/NAF?operation=searchRetrieve&version=1.1&queryType=pqf&query=@attr 5=1 @attr 1=1012 2016-05-16&recordSchema=marcxml&maximumRecords=20