The "editInPlace" code currently requires a logged in user for processing. BTW, remember that we do not encourage its use since it pretty much is not secure.
It should not be terribly difficult to make a plugin that allowed anonymous tagging of objects, though. You just have to supply a form to the object page and process the results on a page load. This approach really is best anyway, presumably you want to do some kind of SPAM filtering on what might be proposed.
Note though, that the object setTags() method does exactly that, it sets the tags to the supplied list. If you want to "add" tags you need to merge the old tags with the new before calling the method.