A simpler, cheaper approach occurred to me: just tie a loop of something (e.g. parachute cord, dynema, etc.) at the muzzle and a loop in the end of the spear-line. To join the spear-line to the gun, thread the spear-loop through the cord muzzle loop to form a reef knot (or invert it into a larks-head).
Update 2008: just bought an Omer XXV & it includes a length of cord to do exactly that. Here are some images of how I used the above technique to rig it. I tied the cord ends with a simple thumb knot (a surprisingly good & useful knot), which I tucked out of the way in the muzzle larks-head, after threading the loop through the small spearline hole in the muzzle.
Although a quality crimper is generally recommended, I am happy with the crimps I made. I used a very cheap, flimsey, rusty old bi-cusped electrical crimper. I made 4 crimps one one sleeve & 3 on the other (following Leadertec's on-line guide). To be sure, I trimmed & then balled the ends on the spearline with a cheap lighter per Miles' excellent beginners guide to rigging; I also dripped superglue into the crimped sleeves.