It also helps that we're talking about rather dense nuclei too. So it's not just a neutron absorbing blanket, but a rather high-performing one at that. Which you need to convert fusion outputs to heat and power anyway. And gold is soluble in mercury anyway, so extraction is already a known (albeit incredibly dangerous) process. Win-win.

yielding several tonnes of gold per plant-year

Mother of god that's a lot to magic-up outta nowhere. At first I thought this would disrupt the market, but it looks like yearly global gold production is around 3000 tons a year. So it would take a lot of reactors to impact the gold market, so... yeah. Reactors really could start paying for themselves.