I've seen an 18-2 with lengths of chain on top of the tramp run diagonally between the inside of the hulls and the main beam. The jib blocks could then be attached to appropriate link. The idea being that with a 4-way system you're going to be moving the leads either out and back or in and forward, so a diagonal line gives you almost the same adjustability, and less clutter

I can't recollect exactly how it was rigged, but it seemed to work quite well.