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.
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.