Tony from the UK sent me this picture of how he arranged the mainsheet traveller. His boat is a Vivacity 21, but the sheet arrangement is the same as on the Alacrity/Vivacity. When I first saw the picture I wondered what the benefit of the rope really would be as I reckoned the block would move almost in the same way as would a shackle on the iron bar. This is Tony's reply:
With the traveller it all depends on how tight the rope is, if it's longer and looser than the one shown then the block will travel further, but I found a length I was happy with and cut it to that length.
Funnily enough, it doesn't seem to travel very far which seems better. If it was tightened up even more it would remain more or less fixed in the centre position. But you are right, it is a similar arrangement to the original.
Now I think the main problem with the original setup is that in a beat you sometimes want the block (or shackle) to be to windward, not just in the middle. I am not sure if Tony's arrangement does make this possible.