DocM,
Although you already know I'm not totally on the P4P bandwagon for many ethical reasons I love you concept here. Might I add to your concept of the idea of using said button or tab as "linking" of some sort? I too don't want to mislead others who might be helpful in getting this started, but I also imagine in your example that certain tests, labs, and exams, what have you, might really be important and useful to any number of Dx's. And so the idea of these buttons being able to "link" anything that you providers care to link to any given Dx might be very useful.
In this way when you run this patients labs, you may need some of those results for either of her two Dx's of Cancer or DM. So you assign button "A" to cancer and get all the things that you assigned to the cancer Dx. Meanwhile button "B" you assign to her Dx of DM and then set up any and all things you would like or need to see for the DM. Am I getting you and are you understanding what I am suggesting, adding or agreeing with here?
The one issue I can see, like all things that are divided and linked together, the usefullness and the quality of such a thing will only be as good as the system or the person doing the sorting and the linking. But if made fairly easy like with quick buttons and the like, this may prove to be extremely useful and a great improvement from the present system we all use. Personally I think this is a wonderful suggestion that deserves further exploration. Great Idea.

I just had a lightbulb kind of moment on your great idea. What if in various places like in the labs section and others, there would be a quick and easy drop down much like in the final section for creating the invoices with the Dx codes. So as you are reviewing a lab, you could put any number of Dx's with it so then that lab will link up and show up for your review anytime you hit the button the corresponds with the Dx's that you previously set them up as??? Obviously for those of you who know your favorite 50-100 ICD-9 codes, perhaps as you type it would give you more and more limited choices and make it quick and easy.
Question, how broad or narrow too??? Still great idea.