Both use-cases are very nearly possible.

At the moment, we change the Owner and Active authorities to require multisig, but leave the Posting authority alone.

The Wizard currently won't let you set up an account with higher weight requirement than the signatories can meet, so the "provably unspendable" part won't (yet) work.

I'll need to check if the blockchain validation disallows that; if it's possible then we should change the "signatories have insufficient weight" message to a "are you SURE you know what you're doing"...