Wouter, you can do it from the wire. You just have your tack line fed to the outside of the front beam and then set parallel to the starboard hull with a bunjee take up. Trust me
I used this system at the UK 2007 Nationals. However, I found it a bit to unreliable especially when you happen to trip the tack whilst gybing!! It's also a bit scary if you get a bit carried away with your new toy and pull the tack out a bit to soon and the spi starts to fill backwards whilst creeping out of the chute prior to the mark rounding. Therefore, I think the double line system suits the two up's better and due to the simplicity of the single line system it benefits the singlehander.
Scooby enjoys tinkering with various elaborate systems I wonder what his preference is?