do you think using magnetic bellboots (like the ones featured in Dover catalouges) could help a horse with navicular disease?

If your horse's navicular syndrome is caused by over-exertion to the deep digital flexor tendon then magnetic bell boots and magnetic tendon boots is likely to improve circulation to the navicular bone. However, if your horse's navicular syndrome is caused by poor conformation, excessive work load, obesity, or poorly shod feet then the magnetic boots will do little to nothing for him.
The best treatment I have seen in horses with moderate to severe Navicular is to have the farrier round the toe off when trimming, adding wedge pads to relieve pressure on the deep digital flexor tendon, and orally administering a vasodilator such as Isoxsuprine to improve blood flow to the hoof.