This week I started to change my clocks to 12-hour format. I made changes on cellphone, watches, and now in my window manager, awesome.
In awesome you need to pass an argument to wibox.widget.textclock() in your rc.lua. I used this:
mytextclock = wibox.widget.textclock(" %a %b %d, %I:%M %p ")
The widget textclock uses GLib to get clock data. A list of supported formats can be found here.