Thank you waterwingz. Unfortunately I didn’t manage to simulate the manual half-full-half-full press. Burst mode sounds ok, but I don’t know how to put the camera in half press mode before using it. The CA-1 mode looked almost ok: after the first short pulse it beeps (without shooting) and on the second (long) one it shoots immediately without beeping. But at the next long pulse beeps again and then shoots. It looks like it needs the short pulse every time. It is useful in other applications because you can set focus and exposure in advance with the short pulse, then wait for the event you want to shoot. When the event arrives, send the long pulse and the shot is immediate. But it works only once. If the following pulse is long, it behaves like a normal shot (decide focus and exposure, then beeps and shot) which is too slow.
I’m really thinking something bad: avoiding using chdk and use a servo motor to mechanically push the button, calibrating it in a way it moves between half and full. Very bad, but in this way I can get 2 frames/sec!