What is the possibility of making this a configurable option?
Assuming its physically possible, and someone actually came up with a reason to have it, probably not too bad.
It should be physically possible, it is with the OE firmware. Pressing the power button takes you to a charging screen, then shuts down when the battery is charged. Since Rockbox reports 100% battery power when plugged in (and doesn't support the charger) there should be another way to access this shutdown routine.
I came up with a reason. I want to be able to access the normal shutdown routine while plugged in. My setup is similar to a docking station, just cheaper, an AC powered USB port with the provided cable.
The fact that Rockbox doesn't access the on board battery charger in this case actually helps me. Leaving it plugged in for a couple of years won't slow cook my battery like it did with my iPod.