Reverse polarity on any of the three power inputs will cause damage to something on the board. (Either the vreg or battery charger ic) SparkFun doesn’t specify which voltage regulator is used in the schematic so I can’t say what over voltage protection there is but as long as you don’t input more than 5 to 6 volts you should be fine. Maybe SparkFun can let us know what vreg is on the board so we know the max voltage on VIN but if it were me I’d say 5 volts max on VUSB and 4.3 volts max on VBAT to be safe. It’s possible VIN may max out at 6 volts depending on which vreg they used.
UPDATE
Per the hookup guide, VIN is max 6 volts. I’d recommend building your own 5 volt voltage regulator circuit wirh polarity protection diodes that connect directly to VIN and ground. That way the artemis is always happy and protected.