The oscillation is gone! In short, the reason was not AQ firmware as I first suspected, but closed loop RPM mode of ESC32's.
I updated firmware to 6.9 and went out to give it a go. The oscillation was still there, I could see tips of the booms shake ever so slightly up and down. The shivers would come and go, sometimes shaking the whole copter all the way down to camera gimbal. I started to think if there was something else that could cause really fast oscillation and then I suddenly remembered someone saying something about tuning the PID (or PI?) of ESC32 closed loop RPM mode and how you should not start with closed loop mode in a new heli because you end up having multiple un-tuned PID's after each other. It's obviously hard to tune them one by one if all are active at the same time. So, I rushed inside to change ESC32 settings.
I tried first with my laptop running Windows XP but could not connect to the ESC32's. No matter what I did all qGroundControl said was "connected - unknown firmware version" and the settings boxes were left blank. I tried qGC 1.3 RC 1 and 1.3 "official", different USB ports, different baud rates but still nothing. Finally I gave up, fired up my desktop PC (running Windows 7) and was able to connect right away. Any idea why my XP machine could not connect? I was using the same cable and same qGC version both times.
I switched all the ESC32's to open loop mode and went outside. Nice stable humm (the motors actually "humm" since I'm running 380kv motors with 4S batteries, the RPM is nice and slow), no oscillation, no vibration, no "twittering" sound from the motors. Nice! Even the altitude hold got significantly better just by reverting back to open loop mode! Who knew!? I still have a lot of tuning to do with altitude hold, it tends to drop at a steady rate. But not as fast as when the ESC's were in closed loop RPM mode. Why's that?
Overall, 6.9 seems to respond slower to stick input and feels quite sluggish compared to 6.7 (both at default PIDs). I might raise the PIDs for agility a bit, but in general sluggish and slow movement is just good for an AP heli in my opinnion. It's just a tad too sluggish, so minor tuning is in order.
Ok, now I can continue building and finalizing the hex and my new brushless camera gimbal. I'll put up some pics as soon as they are ready and tested.