Thanks Truphao.
My understanding is that Sole-Props have always been able to adopt a 401k on 12/31/YY and immediately make their "election" (for up to max deferral), subject to the finalized calculation of their limits based on finalized calc of Considered Compensation (which may actually happen as of 10/15/YY+1). I think the reasoning is that their income is not established until 12/31/YY, and even then subject to many computations that take place after that date. I'll leave it to the forum to either confirm, clarify, or correct my understanding and explanation on that point! Anyway, on that basis, I think the principle would be the same for the proposed SHORT PY--with the compensation period duly defined as above.
One additional thought in support of my original proposal: it appears the intent of congress was to make this provision available ASAP because they explicitly made the effective date "after 12/29/22"; if they wanted the impact of of the section delayed to 2023 PYs they could easily have made the effective date "after 12/31/22". However, code writers are clearly not practitioners, so it didn't occur to them that it might present a challenge to implement the new rule for 2022 Plan Years.