There is a pretty good article on the YTWL circuit written by Nick Tumminello. He actually explains why it is more difficult to perform them on the swiss ball.
In addition, he explains that the W isn't necessary, and he swaps it for a new exercise called the pivot prone. I'm a baseball pitcher, and I have been doing these without the W for years, but now I'm adding the pivot prone to my routine. I also learned a few techniques, such as reaching out on the Ys to get better muscle recruitment. I've never gone about 4lbs on these though, and I do each letter straight through before moving on to the next.
All in all, it's a pretty good article, and I think when people give it a try they will make the switch. Check it out here:
http://nicktumminello.com/2009/07/a-...improved-ytwl/