I just posted about this issue the other day on another site I think, but a shackle is a poor idea for several reasons. A wildly flogging sail, with crew on the fore deck, can turn very nasty, (I know).
And, some years ago, when sailing very close to the lee shore on starboard, I tacked around to port, and when the jib came across, the snap shackle firmly attached itself to a stay. I was flying headsail only, as it was absolutely howling that night, and when the crew rushed forward to engage the shackle, (without the presence of mind in the situation of not releasing the halyard), the port wind whipped the boat around, and run aground. Mike Tyson could not have released that shackle. I have also had screw shackles hang a shroud as well, but it didn't end that stupid.
Now I might add, that I was sailing the ditch, so a lee shore is ALWAYS an issue. There was nobody out that night, and I like sailing the "extremes", and it annoys me intensely to see perfectly good sailing vessels motoring along, when it can be done simply enough with a sail.
But no shackles on them...