My ~1971 IYE Tornado mast has a AL oval box welded through the middle of the mast for the jib halyard. There are 2 sheaves in the box to redirect the jib halyard down the luff track. The same could be done for the spinny. I've run a spinny many times on this old mast, and it hasn't folded, so the idea is structurally sound, but it would be a poor idea for a spinny halyard, IMHO, due to mast bend causing the halyard to chafe against the main sail bolt rope.

Randy Smyth addressed this on one of his boats (an 18HT, I think) by simply running the spinny halyard down to a block 12-18" forward of the main beam on the spinnaker pole. It was more exposed, simple, and it avoids creating turbulence on the low pressure side of the mast, where it is more costly.

And then there is the 'down the forestay' approach. That's extra friction/wear and slow hoists/douses on a sloop rig. I'd only consider that on a unirig, and then there is the issue of extra hardware/distance/stretch/weight. It's not exactly KISS.

--Glenn