Custom page fields

I am building a custom navigation MVC widget in which I'm trying to filter on a custom field that I've added to the pages.  Below is how I'm  selecting the pages to be included in the custom nav, but how do I add a filter for the custom field?

var pageManager = PageManager.GetManager();       

var pages = pageManager.GetPageNodes().Where(n => n.ShowInNavigation == true && n.ApprovalWorkflowState == "Published" );

edit: This is for version 9.2

Anyone??