Yes, the etch for VBAT (Pin 3) on the RTC runs under the chip, so it is not practical to try and cut the etch. The best way is to unsolder and lift the VBAT pin and wire directly to the lifted pin.
Hopefully it is easily doable for you, but let us know how things work out. Thanks.
Suggestion: next revision, think in left the vbat independent and connected to vcc through a pcb jumper. If one wants to use external battery just cut the jumper (knife) and solder a wire in the jumper pad.