You don't need to give posting authority though. You can sign one-off actions and require interaction with steemconnect at every action, no? Seems like a great fit for steeminvite.
e.g. steembottracker and smartsteem both follow this pattern.
Unless I'm missing something.
The only transaction to sign, create_account_with_delegation, is not supported by them anyway.
Oh, well that certainly kills it :)