The issue with canonical IMO is that app canot change the link. If busy.org change his url scheme or busy.org become busy.com, we would still have old style canonical url from others apps. I would rather recommend 3p apps to use only app.
For the standard that's something we can do at Steemit Inc. We can imagine a repo with all standards. Similar than what was proposed with SteemScript https://github.com/busyorg/steemscript. With that in place, 3p apps would have the possibility to PR change on the repo to update url scheme for example.
RE: Steem improvement proposal : Canonical links.