Great idea. This will save tons of blockchain storage resources.

Marker syntax : [//]: # (hive-sig:v1:{author})

What's the purpose of the version field in the marker? What could break here if it wasn't mentioned and we actually had a major change?
Why not make it optional and use the latest version available find in posting_json_metadata if ommited?

The same goes for the author field. The author of a post or comment is already known. It should be optional and only mentioned if one want to use another account's signature.

KISS