That's an interesting idea and useful if the traditional prototyping route is too fragile. But with predefined footprints you can end up running out of footprints with much of the board unused. For instance, if you have two chips of the U5 sort, placing one has basically filled that board.
Perhaps you could have a 'master' PCB where many, if not most, footprints are actually for a daughterboard (are we still allowed to use these terms? I lose track...). Those are a standard pattern and then you just choose the daughterboard that suits your chip - all it would be is a SMT to whatever interface (pin header, castellated edge, etc). A bit more expensive to start but cheaper overall as you're not wasting so much.
One other thing: the pledges suggest I could order two boards for $15, making them $7.5 each, or five boards for $50, making them $10 each. I would have expected them to get cheaper the more I buy, not more expensive!