Floating Object Illusion with Moving Camera?

Hello,

I have a question about a certain effect I'm trying to achieve with FCPX. I want to make an object appear as if it's floating. I know the way to do this against a not-moving-background would be to shoot a video of just the background, and then shoot the levitated object with string or something, and then layer mask out the string against the background video that was shot.

However, how would you do this with a moving camera? For example a tracking shot or filming handheld style, how do you achieve the same effect? Is it the same way, with a background and the object, and you just have to hope that the two videos you shoot line up perfectly? Or is there a more efficient way to do this?

Any help is appreciated. Thanks!

John