From a Bosuns Chair attached to the existing Genoa Halyard (on the starboard forward masthead sheave), you can get to the forward port side sheave and feed a thin messenger line and a series of crimped on 1/4 ounce fishing weight over the sheave and internally down the mast. The aft rake of the mast should let the weighted line slide down the aft side of the mast clearing the spreaders and exiting at the mast base sheave.

After feeding in the string and weights and seeing or hearing them at the bottom sheaves, drop the remaining messenger line to the deck. You can come down off the bosuns chair and using a soft thin wire snag the weighted line and pull it out the underside of one of the port side mast base sheaves. You might have to bounce the weights up and down to find and snag them.

The process for installing a spinnaker halyard is more complicated in that where the spin halyard should have its own exit slot / block cut through the forward face of the mast below the masthead with a clean line UP to a swivel block attached to the bail on the forward top of the mast head.

Youd need to find the right fitting and cut a slot for it into the mast without hitting the conduit carrying the wiring harness up the mast. The line of pop rivets just off center to port on the forward face of the mast hold that conduit in place.

I have installed the second Genoa halyard and sometimes have actually used it during a race. Id be the first to say that a back-up Genoa halyard is nice to have, particularly as a racer -- it was not a particularly easy task.

Once you have the messenger line run, you should be able to attach the new halyard to the messenger and fair-in the connection with just enough duct tape to allow it to feed into and over the sheave and down the mast.