is there any way to make CHDK set some voltage (or current) on USB port?
No, not in the standard CHDK build.
I think EWAVR experimented with pulling the USB data lines high and low.
it could be used to trigger external flash
Anyway, it would be of no use to trigger flash unless it was during a long exposure.
Somewhere in CHDK you can disable preflash, this allows you to use standard slave flashes.
Alternatively, this device allows you to connect external flashes :-http://www.gentles.ltd.uk/gentstereo/options.htm#PRO
or control other equipement.
I (and another group member) have been thinking about creating a product that allows you to do that by flashing an LED.