I like the utility of the canonical property.

The app tag is not particularly scaleable, and adds several brittle complexity obligations to all clients.

1. The blockchain doesn't know jack about apps.
2. The universal url schema is either fixed for all time, or everyone in the steemverse needs to change their schemas

I'd suggest we start with both a canonical and an app_account (that is an account associated with only the app that assisted in creating the post).

At some point in the future we can either:

1. agree on a kind of "meta schema" that will allow the account app_account to publish a configuration set for in a special tx; so that all clients can generate the correct urls by pulling the most recent tx for that app_account.
2. the app_account could publish special tx with a javscript function in the body that would take a post and transform it into the correct canonical url. (this is a terrible idea for all the obvious reasons)
3. have a standard js lib in an npm style repo somewhere that everyone has access to, with the functions describe in #2, but vetted by the publisher (this is terrible for a mostly different set of reasons)

If we get sufficient community support, and add in the concept of trusted app_accounts (with some additional complexity) any of the above would get the job done 'well enough'.

Oh, I agree this really does need to happen. For all our sakes.