It's good to use a Spectra/Dyneema span, certainly, as not only do you spread the load, but you have easy adjustability of the halyard thimble position - before the alpine butterfly knot has pulled up so tight that you can't shift it!

But as you say, the bolt rope makes things difficult. My best effort is this: because the load has been spread, the bending moment at the two attachment points has been decreased. Boring a small hole through the yard horizontally and on the median line won't weaken it in the vertical direction, and will have little detrimental effect in the horizontal direction. You'll only need 5mm line, at most, as the breaking load is so high, so you should be able to push it through a 6mm hole. You might want to add a small glass/epoxy bandage in way of the holes.