It's an excellent hack, but it also just feels so wrong, at least in some cases. Most users (of a URL) won't care about the tags, so why send them? Maybe User-Agent differentiation is an answer? Define a string which indicates the type of response that rkt is looking for?

Are there any examples of publicly standardized User-Agents?

At least it would allow one to take meta tags to specialized binaries out of their webpage markup. Well known URLs is a decent compromise, but potentially breaks existing applications and extends across all applications.