It's what I'm doing on my P5 for next year. Got the Cateye buttons and everything. Isn't your Shiv single ringed up front? If it is (like my P5 is going to be) you don't even have to hack up a set of proper shifters. You can use the SW-R600, which is a lot cheaper at about 60 quid.
Something for the Shiv next year?
http://dinosarti.com/blog/2013/5/23/hacking-the-heck-out-of-my-di2-9070