So if you trigger with USB you have to work with a script...
In the german forum there's a script
"Belichtungsreihe Fix-Focus", in german language but it can be translated easily...it's not with USB control yet.
(for DigicII, get_prop 205 have to be changed to 206 for DigicIII)
The Script does focusing only one time, then the focus is fixed.
It's an extension of a script originally written by CHDKLover and msl, maybe they can help you ...