You might have a problem near those iron mines Pete.....joking aside...shimano SPD does come in two variants. ... single release sh51 and multi release sh56....I put multi on winter shoes so I can unclip easier...this however does create bad habits and some retraining in the spring.
One concern I'd have is that those small magnets seem to really prone to corrosion. We use similar small magnets for trim pieces on tanks that we have to remove for maintenance. Any that get exposure to humidity start to rust and swell pretty quickly. My guess is that,with the conditions seen riding, they would need pretty frequent checks and maintenance. If they start to swell in the pedal, they'll likely be a hassle to remove, and leave behind a bunch of small magnetic filings that are a pain to clean. We coated all of our service mags in epoxy because of how quick they deteriorated.