I agree with all comments - that looks like a dangerous approach that is not likely to work. I may be at my boat later to take pic of forestay fitting, and I have a circa 1991 Harken furler (that may be like the part the OP got used).
On my Z-spar mast, the stay attaches using an eye fitting at the top of the stay to an aluminum bracket embedded inside the mast extrusion. That (cast) aluminum fitting has a clevis pin that the eye attaches to. The stay goes through the furler foil extrusion and supports it, and of course the mast. The lower part (drum and swivel assembly) of the Harken furler acts as a turnbuckle and is tightened to tension the stay, similar to what would be done with a standard turnbuckle on a hanked on jib. To get the mast adjusted, that has quite a bit of tension.
I can't figure out what GregP intends to do with that assembly. If the idea is to use the green halyard to support the stay, I think that'd be very dangerous - way too much tension. I do see a stay in the photos, so maybe that would go through the furler extrusion (need pics of lower assembly of that stay). I see what looks like welding above the fitting. It is possible that the cutout in the mast that had the aluminum fitting for the forestay eye was removed and was welded over (no idea why, and what that hound and all the other stuff is for). The green line looks very light for a halyard.
If your ebay Harken is complete (pics of it would help) and has the swivel at the top, you may not be bale to get that halyard high enough to pull it up, which also hauls up the sail. On my mast, the halyard sheave exit is a bit below the fitting for the stay's eye, and must be in line with the stay (more or less) and tensioned so it is not too tight (or else the swivel doesn't turn) or too too loose (or else the halyard wraps around the stay and prevents the sail from furling or unfurling). I suspect you will not be able to get that with the side mounted halyard.
Do you want to just be able to just remove the sail or the whole furler? If the sail, and you have a complete Harken, that is not hard, as noted - if you have a proper halyard (maybe you don't) then you just lower the sail, the top swivel slides down the extrusion, and the bolt rope comes out of the slot in the furler. A bit harder than a hanked on jib, but not hard. But if your sail has a protective cover on the luff, why not leave it furled - that cover would protect the sail.
Does not look workable to me, but good luck.