I don't really know but through the power of google I bring you this from the hobie web pages.
Also, be sure that the knot tied to the ring is very low profile. A long bowline knot will hit the mast head before the ring gets to the hook. If the ring has a small loop at the top... The line should be passed through the loop and a small knot tied. The knot (when ring and shackle are afixed to the sail) should be facing the mast. This tilts the ring closer to the mast. Then (before attaching halyard shackle to the sail) spin the haylard 3 or 4 times clockwise (looking down on the shackle). This "pre-loads" the halyard line and causes the ring to swing back towards the hook. Keep the boat into the wind and hoist. Should lock easily. To release... fully release the downhaul and outhaul. Partially feed the sail up the luff track. Hoist with the halyard to the top till it stops, hold... rotate the aft of the mast base to starboard, hold the mast rotated, ease the halyard a few feet before releasing the mast. Lower the sail.
The buntline hitch is a small effective knot but you may not like it here because it doesn't like to be untied. If you were wanting to leave the ring on the halyard it would be worth a try.
And I found this, something called a topsail sheetbend, which would probably work if you have the two loop ring.