It wouldn't be a successful migration to a new PCB tool without introducing a new hardware bug, would there? ;) That's why pad of the SERCOM module is connected to RX of the BT module. However, pad cannot be configured as SERCOM TX, only pad and pad. To solve that we could
a) configure the pin connected to the RX of the bluetooth module as an input (high impedance), configure one of the unused pad pins as the SERCOM TX and run a wire between them.
b) emulated UART in the firmware like the SoftwareSerial library of Arduino. Since we're only sending data, this could work out all right.
Going for a would be the quickest solution. However, I'll give b a try, keep the PCB clean and learn something new.