Indesign script that applies a master page to all empty pages in the document.

Apply Master To Empty Pages.jsx

// https://gist.github.com/joonaspaakko/f16b47be887a2058cd0c0ac6e7ab0343

// The script doesn't check for page items inside the current master. 
// That means the current master could be filled with all kinds of graphics and this script considers it an empty page.

var master;

if (app.documents.length > 0) init();

function init() {
	
	var doc = app.activeDocument;
	
	var masters = get_masters(doc);
	dialog( masters );
	if ( master !== false ) {
		
		var pages = doc.pages;
		for (var i = 0; i < pages.length; i++) {
			
			var page = pages.item(i);
			
			// Not a single page item in sight...
			if (page.pageItems.length < 1) {
				
				// Apply master to the page.
				if (master === "[None]") {
					page.appliedMaster = NothingEnum.NOTHING;
				} else {
					page.appliedMaster = doc.masterSpreads.itemByName(master);
				}
				
			}
			
		}
		
	}
	
} // init()

function get_masters(doc) {
	
	var everyMaster = doc.masterSpreads.everyItem().name;
	var masters = ["[None]"];
	
	for (var i = 0; i < everyMaster.length; i++) {
		masters.push(everyMaster[i]);
	}
	
	return masters;
	
}

function dialog(masters) {
	
	var dlg = new Window("dialog", "Apply Master To Empty Pages.jsx");
	dlg.orientation = "row";
	
	var dropdown = dlg.add("dropdownlist", undefined, masters);
	dropdown.selection = 0;
	
	var buttons = dlg.add("group")
	var btn_ok = buttons.add("button", undefined, "Apply Master");
	var btn_cancel = buttons.add("button", undefined, "Cancel");
	
	btn_ok.onClick = function() {
		master = dropdown.selection.toString();
		dlg.close();
	}
	btn_cancel.onClick = function() {
		master = false;
		dlg.close();
	}
	
	dlg.show();
	
}