There's quite a lot of different methods of giving one-switch / one-button users choice. Perhaps some yet to be invented. Don't feel constrained by the conventional column/row scanning, is my personal view.
As long as you are sticking to the competition rules, you should be good. Importantly, make your game accessible to those one-switch users with significantly slower reaction speeds, and those unable to hold / mash / press a large number of times.
If you want to offer a lot of settings to the user, bear in mind, these can be very confusing for some, or complete drudgery. You may want to be able to shield those off, just for helper access (bear in mind some helpers will be able to use conventional, faster controls, but some won't). Some users may want to get at everything with their switch control.
Do feel free to post up more.
