Splicing a Y in 12 strand?

[Gashmore]

We are lowering and retrieving a 1,000 kg package of geophysical instruments into a cave. Total hoist is about 30 M. We have rigged a block on an A frame out over the top ledge but there is a shelf about half way down that we have to get around. I rigged a line to a block on the opposite wall for a control line but there is a second shelf further down that fouls the control line if it is attached at the load. The current procedure is to lower it about half way to the shelf then tie on the control line but we have to retrieve the package every 10 days and climbing out on the A frame to put on an icicle hitch on the way down and again to remove it on the way up gets a bit harry. I was hoping for a splice that could pass over the block.

We are using the Dyneema because of the low stretch. The package hangs a few inches off the mud bottom so we need to know exactly how much line we have out.