Master Pages. Provided I understand the question correctly (I'm sort of blindly hacking my way through the jungle that is InDesign right now :D).

I was poking about the library's ebooks (we have 3 ebooks for CS2, but I'm hoping it'll be similar), and just ran across a section on making graphics transparent with Multiply ... that looks like it might work, since the spot where the numbers will go on each page is white.