The main restriction I find with Master Pages is that you cannot modify the MP contennts or actions when you associate a MP with a "normal" page.
MPs are really great for adding consistent elements such as navigation buttons, backgrounds, running page titles etc. They are not so great when variable (not variables) elements are involved.
Bear in mind that you can use several MPs in a pub, and you can "nest" MPs. This can sometimes help in complex pages.
OK, now back to your specific problem, which I hope I understand correctly. I would not use a MP to do what you want to do. I would add a frame to the page (
not to the MP) and use the On Show trigger to activate the actions associated with the frame.
If your checking of the variables is complex, add a series of IF statements (or an appropriate script) to identify the specific prerequisites.
If you have a number of different prerequisites that need to be checked, design a separate frame for each check. Copy these frames to a gallery. You can then paste these back to pages as you need them.
HTH