I don't think there's an actual API for policy assignment rules.
Technically you could 'Roll your own' through the API with a
policy.assignToSystem( system.find( "yourTagCriteria" ) ) kind of query.
Also do you have the ePO.PAI.Explorer tool? https://community.mcafee.com/docs/DOC-2931 It's very good for browsing all the avaliable commands.