Is it safe to add a trickle charge current to Balboa control board? The charger would connect to the control board via the pins labled VBAT and GND. When charging, the USB connection would be disconnected, the power slide switch and the power latch circuit would both be off.
Using a charger with the Balboa 32U4 like that should be okay; VBAT is a direct connection to the batteries. As you indicated, it will probably be safer to have your robot off while it is charging.