My bank will send me a warning that an ACH is pending, and I need a certain amount in the account by 10am the next morning. That's plenty of time to move funds into place from a neighboring account (as long as it doesn't involve selling shares). And on the other hand, it's an interesting benefit that the requester of the debit doesn't see this happening.
I think the solution is an explicit ACK. We could add an optional pingback url as part of each transaction message, and let everyone else deal with building the infra.
I have a feeling it's a lot more complicated than that. $39 trillion a year is a lot of transactions to track live status for, so I suspect this is both complicated and valuable.
What the heck NACHA -- impress us with your ability to build the next gen ACH system! Oh right, unless we're talking a physical structure, that's never going to happen.