Correct. The menu is limited to three items max (currently Game A, Game B, and Clock), so there was no available way for me to offer this as an option.
I think I have access to a variable that lets me know if the user set 12 or 24 hour on the Playdate itself, so I could in theory read this to adapt to the same setting.
I chose to stay true to the original G&W device that was using AM/PM with a 12-hour display.