There are a couple ways to do this already:

• Custom operations allow any user to post arbitrary JSON to the blockchain that's ignored by existing tools (but the poster still has to have an account with SP to permit the bandwidth usage).
• Comment options operation allows a poster to opt out of rewards for a post, but a top-level post still counts against the poster's limit. This should be easy to change.

For a Twitter clone it makes more sense to use custom operations.

FWIW the comment options operation has been in the blockchain forever, but the code for the steemit.com UI is currently in testing.