This is going to be tough to do without removing at least one end cap. The outboard end would allow you to see (maybe reach) the loose end, but the tackle itself is likely anchored to the gooseneck end cap, or to the boom itself towards the mast end.
How difficult this is going to be depends on how far you pulled it though before you lost the messenger.....
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"