An adjustable splice?

[benz]

Thanks, everyone--these forums are such a great resource.
What I'm doing is trying to rig a tightrope between two tall cliffs (not for me to walk--I'm not that crazy), so all the materials must be light enough to carry several miles uphill in backpacks. My client doesn't want to be limited to walking just one gap, which having a rope of fixed length would do. He wishes to start small (200-foot span) and increase the gap gradually without having to buy a new piece of rope each time, since 3/4" Amsteel is hardly cheap. Hence the need for an adjustable splice. I'll do some math and see whether losing 12% of the rope's strength still leaves him the large safety margin he (understandably) wants. My own safety margin is to stay safe on the cliff, well back from the edge.
Thanks again! Brion, do your clients ever look askance when you say, "So, we'll heave it up with a whopee, then slide a yippee around the foot...."
Ben Z