As luck would have it, I'm working on drawing and moving rectangles for setting my shot metering areas. I noticed that if I move a rectangle over the Canon AF box, it doesn't redraw itself. So I was able to write a script that keeps the Canon overlays invisible.
You have to turn everything in CHDK/OSD off, as you've probably discovered already. Here's the script for you to try (also attached)
--[[
@title Clear
--]]
while(is_pressed("shoot_half"))do sleep(100) end -- wait for shutter up
exit_alt() -- clears bottom line
repeat
draw_rect_filled(0,0,359,239,256,256)
coroutine.yield() -- equivalent to sleep(10) but more efficient
until is_pressed("menu") -- press menu to exit
enter_alt() -- shows script ended
