Thanks for your tip, it confirm the behaviour I supposed: CHDK can't override shutter speed with something in hot shoe.
In G9 hot shoe there is a little switch that, I guess, activates this safe mode.
That's my test:
manual mode
Canon Tv: 1/100s
CHDK Tv: 1/10,000s
with external flash mounted, Tv: 1/25s
without external flash, Tv: 1/10173s
F.random