I've heard mixed things about magnets. They've worked for some, done nothing for others. Supposedly they increase blood flow to an area, although I don't know enough about ringbone.
It's high ringbone and she's not lame on that foot at the moment, but she does trip over her toes sometimes. I've had her toes trimmed short and square to help with breakover and that seems to have helped, but I want to do everything I can to make her comfortable. I think she got ringbone because she wasn't trimmed properly before we got her and she was run on hard ground with her feet in that condition for years. It's a frustrating situation because she's such a nice mare and this was avoidable.
If it is high ringbone, a magnetic bellboot (at least the ones I'm familiar with from like 10+ yrs ago) won't really address that issue ANYhow. They used to make magnetic pastern wraps or whole-ankle-and-foot boots, dunno if they still do but that'd be more geometrically appropriate for your problem if you really want to try magnets. (wait, I just googled, various companies do still sell magnetic pastern wraps or cuffs).
Personally I think the chance of the magnets making a difference in this type case is very very low, but, doesn't hurt to try it I suppose if you can afford.
Your best bet is probably to continue pursuing as excellently-correct trimming (and shoeing, if she's shod) as you can, since IME that tends to make the biggest difference.
Thank you for your input, everyone. I think I'll pass on trying this, then. Hopefully the problem won't progress too fast. I have her on a MSM supplement. She catches her toes now and then, but other than that, her stride is confident and smooth. Our farrier suggested that we keep her barefoot.