I don't know how well mattress pads work for quilts. But personally, I love the idea of repurposing old stuff to make new stuff. It's wonderful on so many levels. Slightly off topic - my FIL grew up impoverished in the Caribbean, and he is so resourceful in using things in ways other than they were intended. He hates to throw out things that could be used for something else someday. He really has been a great example to me.
If/when you use the mattress pad, Julie, let us know how it turns out.