We're replacing the floors in my living room and I've started giving some thought to installing an electromagnet underneath the floorboards so I can do a light/heavy trunk trick there. I realize that this sounds crazy, and I do actually already own a Malloy "Gravity Box", but if I actually go thru w/ the construction, here's the question...

Should I be at all concerned about having a powerful electromagnet around? Obviously, it won't always be turned on, but do I need to be worried about using it if people have plates in their head, or pace-makers?

A small low voltage electro magnet will often do the trick. It need not be especially large.

I don't believe it is too much of an issue with all the electromagnetic waves already around us.
You need to have thequietest you could find and also you should have easy access to replace/change/fix the magnet in case something goes wrong.