What you're describing sounds more like a stay than a halyard per se..
But in any event I think your fears are unfounded.. esp if you're planning to take this 'halyard/stay' to the top of the rig. I'm going to assume you're doing this outside of a roller furling setup? Are your current halyards internal or external?
A couple of thoughts:
If your halyards are currently external (ie, the main halyard goes up, across two sheaves and down the front; the jib halyard goes up, across two sheaves and down the back) I'd give some thought to converting to internal halyards. That would free up the second forward masthead sheave for a second (internal) jib halyard, eliminating your concerns altogether and avoiding having to hang a block/loop whatever on the mast somewhere. The halyard lead would also be correct, ie under the forestay proper.
The added advantage would be a quieter setup with less halyard slap in a breeze. You'd need to add three exit slots but that's a relatively easy job, as is 'fishing' the (now internal) tails out through the slots. This involves installing some of these:
Isomat Halyard exit plates
If you do this, stagger these exits at least a foot apart, and on opposite sides of the mast to spread them out a bit. I'd also suggest having the exits at least 3-4 feet above the deck to ease the leads to turning blocks if they are to be led aft. If the winches/cleats are on the mast obviously they'll need to be above those.
I do think you'll have trouble getting adequate luff tension on the 'storm jib' that is essentially flying free...