The trillion dollar coin idea is a good one, and completely legal, and the idea will come up again if there's ever a Democratic president. And no, it's not inflationary in the slightest (the inflation, if any, would originate with the government spending in the first place, not in how that spending is accounted for behind the scenes).

It's just a way to say the debt ceiling is dumb, that when Congress appropriates $X then that also comes with inherent authority to spend $X.