Hi Jim G, the system you may be thinking of is where there is a close fitting wire down the rear edge of the mast and as you say the sails are hooked in place. No need for a groove in the mast though.
I have used both above methods and would suggest that if you have the groove already in the mast use it as this is a good system.
Re making a new mast.
If not go for the piano wire or carbon fibre 1/16 dia rod held in place with small split pins which are glued into holes drilled into the mast. Another version of this is to find some very small brassscrews (no more than 3mm across the head) drill and screw into the mast aligning the slot heads. When in place pre-solder / tin the screw heads and solder a brass wire into the slot heads.
Then dress maker hooks can be sown into the luff of the sail. Keep the other part of the dress hooks as these can be used to hang the jib on at its leading edge.
These systems are fairly unobtrusive and ensure a good shape to the mainsail.