Fun fact: construction on Tosa was delayed by two months due to a labor strike. I wonder if that would've been enough to put her close to completion - they might've scrapped Fuso for the treaty limits instead.
unfortunately not, she was five months before completion wher construction stopped, two months would have been not enough.
And, on another side note, the US deputy at the washington naval conference asked Mutsu to be scrapped, despite she was completed more than one month
before the conference was called, probably to keep Japan behind the US themselves and the UK with the 16-inch gunned battleships; as the US navy was granted permission to complete three of the four Colorado and Royal Navy got the approval for the two Nelsons the two 16-inch Nagatos might have been seen as a potential threat.
Mutsu was eventually saved as her construction was funded with school childrens and popular donations (and Settsu was decommissioned instead), but for this same reasion i highly doubt that Japan would ever ben granted a chance to complete Tosa, a third 16-inch gunned battleship even more heavily armed than the US and Brit counterparts (8x16 on Colorados, 9x16 on Nelsons but TENx16 on Tosas) would have definitely been out of question.