This wouldn't work because theoretically you could end up with a completely discharged battery and that charge switch being "off" the unit will be dead. But on the other hand when the unit is not powered, the software setting of "charge off" can be inactive and you could start charging with unit off.
Assigning a hardware button to "charging on/off" as
@tolis626 suggested is a great ideea, but please leave the "Bass" switch alone...
I like it and I use it often with older thinner-sounding albums. A long press on the BT "previous" button would be best for this.
@FiiO is it possible to assign a long press on the Bluetooth "Previous" button for "charging on/off" setting in a future firmware?