Noting your status as a Rockbox developer I hesitate to ask this question ... but is that what *does* happen, or are you suggestion that this is what you think might happen?
It's just that I looked at doing this mod a while ago but lost my nerve (also I obtained an H320) and I saw nothing in the DataSheet that made me thing that what you describe would occur.
I just report my experience after trying to do the following mod.
Of course I "might" have done something wrong (I doubt it) but this is what I get with the following wirings on my H115 (not H120/H140).
For me it works, but currently you won't have any software control over it plus you will have to press a button if you want the unit to not go on USB mode while you charge it (either from USB or AC).
Also the size of the BQ chip pins make it very hard and risky. After I removed the wirings some excess solder short-circuited 2 pins on the BQ chip and the unit wouldn't charge at all. I had to battle with it for 2 hours until I manage to remove the excess solder from those small pins.
Until someone finds a way to control it with a GPIO pin I don't think it deserves the effort. You can have the same effect just by connecting USB pin1 with (+) from the AC adapter jack.