Robi, what are you trying to accomplish? Are you saying that the "normal" trapeze system you got with the boat was not adjustable while on the wire and you are trying to figure out a simple way to make it adjustable without adding any parts?
Exactly you cannot adjust it while on the wire.
Robi, it looks to me like the set-up in the Murrays catalog that was referred to is also adjustable in the way you are wanting, but also provides the upward tension that several people have described.
My boat has the arrangement that you are describing, but after reviewing the posts here, I can see the advantages of having the bungy attached to the top of the ring rather than the bottom, and think I need to make some modifications.
I don't have a lot of trapezing experience, but I don't find it real easy to get in and out of the trapeze with my current arrangement.
Mark.