Yeah but for 90%+ of transactions, if you are being paid by a company, you can almost always request a paper check instead of an ACH transfer (sometimes with a fee). In that case they either have yet another account for check writing (which won't be "secret") or they give away their "secret" ACH account.

I'm not proposing this as the solution to fix the extremely outdated ACH/check system, just relaying what I read about what some companies do.