I also agree that this would have made anything like the OTL WNT impossible - all the major powers would want their own 20" gun armed ships. Even if it would soon be evident that they would be quite inefficient ships - but politics is politics and hasn't necessarily any tight bonds to realities and other disturbing factors.
Absolutely it would have.
And the Conference was contentious enough as it was.
It's important to remember that the WNC didn't occur in a vacuum. It was a product of much wider geopolitical competition as well as domestic pressures. The Treaty was about much more than just naval armaments. Just take Britain - since it is her dreadnoughts we're concerned about - she OTOH was up against the clock, eager for a treaty before a) the Anglo-Japanese Alliance expired in 1922, and b) the U.S. completed its 1916 program buildout, which would really strengthen the American hand in any negotiations at that point. Likewise, Britain as facing not only rising domestic unrest thanks to the poor postwar economy but also struggling with rising independence movements in Ireland and India, which also were putting demands on the Exchequer.
Up against that, however, was Lloyd George's expectation that the Conference would fail - that it would open to great fanfare, and rapidly decline into disagreement and inaction (which is a key reason why George himself declined to attend). The British had done little thinking and less planning at this point - the Committee on Imperial Defence was effectively on hiatus for about two years, in fact. So when Hughes opened with his massive disarmament proposal, it caught the British off guard. The Americans, it turned out,
had done quite a bit of preparation (and, uh, code-breaking). Now they were forced to put together more concrete proposals for naval reduction on the fly. Maybe the P.O.D. here is for George to take a more aggressive posture, insisting on more planning, and attending in person as leader of the UK delegation. This might have opened some room for a better British outcome with the WNT.
Even so, there would be limits to what Britain could obtain. And even with a more active "building holiday," there's a risk: What happens if a subsequent British government declines to actually build up to what's allowed? A longer holiday at least freezes your competitor states (both of whom seemed more keen to build up) in place, too. Britain sure as hell wouldn't get that 20" gun ship without major concessions. A couple G3's might be possible, though.
(Of course, poorly as Britain ended up, it was actually Japan that had reason to come away from WNT feeling the unhappiest, and not without reason.)
But at least those long and fast hulls would have provided excellent basis for conversion into aircraft carriers.
A deeply ironic but very true advantage.
