If I might emphasise the comments of intervening posters ..
It is not up to the pilots to ensure compliance with 25.903, 33.76 or 25.1455 any more than it is their responsibility to ensure compliance with 25.121. The aircraft met the requirements when it was certified and WAT conditions will not invalidate an Airworthiness Certificate.
Of course not .. however, it IS up to the pilot to ensure that the aeroplane is operated in a manner which is not inconsistent with the Standards embodied in the design (eg Part 25).
The minimum legal climb gradients that a pilot will be required to meet will be established by a number of variables ranging from OpSpecs to country of registration.
.. and that will be the page in the AFM which details the WAT limits.
As long as there is approved data in the AFM (again, I have data for 0.0% 2nd segment OEI operation) and you do not exceed a published limitation in the AFM, you are legal for a Part 91 flight.
That may be the case (my bolding, above) but there will be another chart which gives the WAT limits .. the how-to-use-it data will indicate that the WAT limit comes first then the other stuff ie, if the second segment charts have data apparently below the WAT limit, that will be for interpolation or similar reasons ..
Climb at 0.0 westbound across the Pacific until you have burned off fuel – that’s legal
That would be constrained by operating rules rather than design rules. However, why would one wish to do that when the aircraft should be quite capable of climbing as per the normal segmented flight path (I presume we are talking heavies) ?
I was told by a captain that the FMC ..
No reason why one couldn't set up such an animal. The sums are fine for the least capable PC .. However, the database amendment workload would escalate dramatically.
I suggest that a more reasonable expectation (unless you have documented evidence to the contrary ..) is that the box will tell you what you can expect to see OEI depending on the weight, Hp, OAT it thinks exists .. but with NIL consideration of any specific terrain .. including SIDs.