A short self-contained JavaScript snippet (IIFE) to apply a tag to multiple TFS Work Items through the TFS Web Access interface. If you don't have a version of TFS that allows bulk application of tags, this can help.

BulkApplyTfsWorkItemTag.js

// Author: Jeffrey A. Miller (@xagronaut), 01/15/2015
// License: Apache 2.0
//
// Description: Apply a custom tag to multiple TFS Work Items systematically
//
// Notes:
//  * Navigate to a TFS query page [YourTfsServer]:8080/tfs/[TfsCollection]/[TfsProject]/_workItems
//  * Identify the Work Item IDs you want to update and edit the array at the end of this code snippet
//  * This code should be executed in your browser's script console (tested in Chrome)
//
//  options:
//    tagName - the tag you want to apply; be sure it matches any existing spellings correctly if it is already in the system
//    workItemIds - the list (array) of Work Item Ids to affect
//    delayTime - how long to wait between browser operations (in case there is latency in loading/saving the Work Items)
//
(function(options) {
	options = options || { tagName: '', workItemIds: [] };
	
	var updateCount = 0;
	var addTags;
	addTags = function(id) {
		if(id) {
			window.location.hash = '#_a=edit&id=' + id + '&triage=true';
			window.setTimeout(function() {
				$("div.tfs-tags:visible li.tags-add-button .tag-box").click();
				$("div.tfs-tags:visible input.tags-input").focus().val(options.tagName).blur();
				$("li[command='save-work-item']:visible").click();
				updateCount++;
				var nextId = options.workItemIds.pop();
				(nextId && window.setTimeout(function() { addTags(nextId); }, options.delayTime)) || alert('Done - ' + updateCount + ' updated.');
			}, options.delayTime);
		}
	};
	addTags(options.workItemIds.pop());
}({
	tagName: 'Custom Tag',					// adjust this to the tag you want to apply -- make sure you spell it correctly, especially if it is an existing tag, to prevent tag 'fragmenting'
	workItemIds: [5001,5013,8002,8063],		// adjust this list of TFS Work Item IDs to include the ones that need the tag added
	delayTime:  2500						// how long to wait time between browser operations (in milliseconds)
}));