Vega, I like the idea of amsteel for standing rigging replacement. How do you plan on getting enough tension on it if you had to rig it up? Do you really sail everywhere with sheet to tiller set-up? That's AWESOME!!!
I'd set up the emergency stay using a trucker's hitch. I've tested it and it works well enough. A refinement would be to sieze a block into the bight of the truckers hitch and shackle another at the attachment point. You can set it up quite tight that way. I have a backstay adjusting tackle too so there are any number of ways to skin that cat.
As Stillraining suggests, wire would be better for a headstay replacement if you want to put up a jib hanked to the stay, but, in a small boat where every ounce and every cubic inch of space counts, Amsteel is worth considering and it beats anything else for ease of use. I would rig the stay with Amsteel and set the jib flying based on what I learned on this last trip. I've rigged my boat twice with Sta-Loks, definitely the way to go IMO, and I say, if that's your emergency plan, better have a spare stay already made up rather than trying to cut to length and attach fittings at sea. If you have the room and the extra weight is no problem, that is probably the best solution. Then of course you have to figure on going up the mast carrying twenty plus lbs of wire or getting it up there some other way. and attaching it with clevis and cotter pins to a toggle at the top of the mast, in the worst possible conditions of course.
I think the most prudent solution for the headstay would be to bend the stay to the jib halyard and hoist it up, then set up the tension as described from the foredeck. I'd set the jib flying from the spare jib halyard.
But that's just my solution for my boat.
Might not work for you on yours.
Actually, we didn't run the sheet to the tiller. We just trimmed the sails so that there was just a bit of weather helm, not too much, and lashed the tiller with a shock cord to windward. The boat would stay on course (Close enough for open ocean work) until the wind changed. On several occasions I didn't touch anything for two or three days.
I don't have an instrument rating
Malie ke kai