You're the second person to suggest that (the first was off-LJ). The trouble with it is that it's fundamentally a physics problem to find a way to perform infinite computation in a finite time in the real world; but the Infinity Machine has no innate knowledge of physics. So you'd have to start by telling it the laws of physics in order for it to be able to use them to test a machine design – and we don't know all the laws of physics yet, so all we could feed in would be our current best guess, in which case there'd be a good chance of it producing output that wouldn't really work, or (perhaps worse) ruling out a design which would.