When multicam is running (after calling mc:start()), you can run any lua on all the cameras with
!mc:cmdwait('call <your code goes here>')
Sine the camera side script runs until you use the exit command, any overrides you set will take effect on the next shot.
See
http://chdk.wikia.com/wiki/CHDK_Scripting_Cross_Reference_Page for the available functions.
The function to override focus is set_focus(), but there are various camera specific issues and requirements. You may need to use set_mf() or set_aflock() before setting focus, or change turn of settings like server AF in the canon firmware.
get_sd_over_modes provide information on which methods your camera supports.
If you do not use MF or AF lock, you will need to call set_focus before each shot.
Note that you may want to use mc:shoot instead of mc:testshots. It is a bit more flexible and doesn't spew the timing test output.
