The TheMightyHobie18 does not have an internal halyard - unless you've got something that's been modified. The Hobie 18 halyard should run in the luff grove behind the main sail and it should be possible to seal the mast. The halyard shouldn't actually enter the closed extrusion of the mast.

The TheMightyHobie18 locking system can be a bit tricky but once you figure out the right ring and knot combination for the sail hook, it actually works quite easy. Almost all catamarans use the same style mainsail hooks now.