A vote for double halyards
Your easy solution for this would be to install an exit block (up-side-down) at the top of the mast, which might be slightly off-center but would suffice for the purposes of safety/redundancy. This line could be led down inside to another exit block (right-side-up) at the base of the spar for the halyard. Storm trysails are often fitted with such a setup.
The slightly more difficult solution would be to replace the masthead sheave box with a double-sheaved one. I did this on my boat. The part was readily available from Dwyer Aluminum Mast Co in Connecticut, came complete, and was surprisingly inexpensive. I widened the slot in the spar and seated this thing into place, drilled it out for the upper-shrouds' through-bolt (actually a pin-- it should not have threads in the bearing area) and now have double halyards for headsails and main. In addition, the double-sheaved box includes a bail for spinnaker and a pin for a (fixed type) topping lift, which could easily be made to hold a block and an external halyard, so that would give you a whopping 6 halyards from the masthead. (I also have an internal pole lift that will function as an inner-headsail halyard as well.)
Even when you have a roller furler, the addition of a second line for some option or redundancy makes sense. The single headsail halyard that is used to hoist and tension the roller-furling headsail gets so little 'exercise' that it very commonly fatigues at the crease atop the sheave. Being able to exchange this duty with another piece of line sometimes will enable you to inspect the last-used one as well as to keep sailing if one of them gets too far worn.
I applaud anyone going to a double-halyard setup, as I consider a certain redundancy in this area absolutely vital to anyone seeking some insurance from losing a halyard. If I may impose a traditional standard, code the halyards as follows:
Headsail halyard #1 (primary)-- port sheave, red fleck
Headsail halyard #2-- starboard sheave, green fleck
Mainsail halyard #1 (primary)-- starboard sheave, green fleck
Mainsail halyard #2-- port sheave, red fleck
This follows what was always done in ages past and intuitively makes sense to anyone who hasn't been taught any other (contradictory) system.
(I didn't get green-flecked line and substituted blue; otherwise my setup follows this.)
