Shimano 5603 STI with a compact double chainset?

[Kerplunk]

I'm currently running a triple chainset but am thinking of switching to a double compact. I know Shimano officially say you can't do it but is there any real reason (other than remembering not to 'overshift' into the third chainring position) why I can't keep the 5603 triple specific STI. Is it any more complicated than just using the granny and middle chainring positions and using the limit screws on the front mech to stop it shifting passed the middle ring. I can see that its possible that I may need to change the front mech as the backplate on the triple may foul the inner ring on a compact.