But this is a far 121.189 Operator requirement not a far25 certifying requirement like the 2,4% gradient.
One needs to keep in mind that second segment WAT is 2.4% and not related to nett. However, whatever gross is required for obstacle or other operational need is then linked to a net capability 0.8% less. Put it down to minor emphasis in meaning.
WAT limits ... a steady 2,4% climb gradient. Can you point me too a reference that says it only momentary?
The physics of the matter is the reference.
As soon at the ambient conditions (Hp/OAT) vary, the steady climb performance will vary ergo the gradient may reduce through the climb. To satisfy yourself on this one, have a look at any WAT chart with different entry arguments.
The reference to steady is to preclude enterprising things, such as a bit of zooming, etc.
Necessary, also, to keep in mind that the regs can't be read in isolation. One needs to review ACs, policy letters and the like to get a story which, itself, varies over the years .. hence one Type will have different rules applied compared to another.