Would you find a dining chair with attachable/detachable magnetic cushions handy?

No, I doubt the magnets could hold the cushions in place and it would likely be a lot cheaper to get regular cushions.

Plus wouldn't it do a number on any credit cards in your back pockets?

I don't know what problem these chairs would be solving. But if the cushions stuck really well, were easy to wash, and were very comfortable... I might try them out.

What about a reversible (flipover) seat?
Cushion one side - Wood on the other.
I eat off a tray on my lap - So neither attract me.

I'm fine with cushions that secure to my wooden chairs. works fine and lasts a long time.

What about the effect on people with metal hip implants? Problems. Also it might affect and/or interfere with credit cards that are kept in wallet in hip pocket.

I don't understand cushions on dining room chairs to begin with. They kind of weird me out.