If the mast
isn't so far forward as to make a preventer ineffective, you can heave to on the mainsail
only, by preventing the boom outboard
against the pull of the mainsheet so it's locked in position and the mainsail
acts like a weathercock. The helm
is lashed hard over as if to round the boat up, but the main goes aback when it tries and so it reaches an equilibrium, at not too different an angle from the usual backed-headsail hove-to position.
However because the boom is long on a catboat, and the mast
is well forward where the beam is narrow, my guess is that you couldn't safely use this manoeuver in big seas, or where the boat might sustain a knockdown.