Sure, if a bank gets compromised you could in theory DOS a clearing house, but I'd be completely amazed if it succeeded. Those kind of anomalous spikes would be detected quickly. Not even imagining that each bank probably has dedicated instances inside each clearing house.

These are fairly robust systems. You'd likely have a much better impact dossing the banks.

Yah, I suspect the banks pay a handsome sum to smarter people than you and me, and they've gamed this out already.

I build such systems ;)