It may not have been "money of necessity", rather I think it may have been "money of opportunity". In the early 1800s, the Napoleonic Wars had put a mixmaster to European coinage; all kinds of new coins were turning up in change. I think this particular token was simply jumping on the bandwagon and hoping his funny little tokens would be accepted as some kind of new legitimate coinage.
If there's a political message in it for anybody, it seems to me to be aimed at the Spanish. "Plus Ultra", meaning "more beyond", was (and is) the Spanish motto. "Non plus ultra" would mean "no more beyond" - the Spanish colonial empire had collapsed to only a fraction of it's former glory. A Belgian token scoring political points against the Spanish would have made sense in the 1500s, but by the 1800s the Spanish were long gone from the Low Countries.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis