I have had success with the OTG Y cable accessing flash storage and charging simultaneously.
At first I didn't think it was working, as the charging animation does not appear. I have been using a blackberry charger 5V 700ma, I have found that if I have the charger already connected to the Y cable *before* plugging into the phone the charging light appears, but still no animation, however battery stats states charging AC. If I connect charger after the OTG Y cable has already been attached to the phone I do not get a charging light.
I put two rather large files on download (to the flash storage) whilst testing this out, as I knew this would usually drain my battery about 20% screen off, went from 66% battery to 76% in 20 minutes.
I am using Linaro Kernel.