Concord / Fargo Aire / Amity made a complete stainless steel metering box to replace the corrosion prone one that came on their original carts. They probably still make it but I don't know if it would be the best solution for grass or very small seeds like canola or mustard. The head wrench around here is exposed to about every different kind of air-seeder there is, as part of his other job. He has many good things to say about the Bourgault meter system. It is much less prone to bridging and a couple other problems the single, large fluted feed meter has.
No real pressure relief per se on my Concord tank. A centrifugal fan just has a tendency to 'slip' when pressure exceeds what the impeller was designed to make. Similar in some respects to what happens to a vacuum cleaner when you completely block the hose -- the motor speeds up and HP required drops.