Good idea on receiving with xbar.
For transmit on pin 7, you could try using SoftwareSerial. It can't receive if the 2 pins you select are not a real serial port. Maybe configure SoftwareSerial to transmit on pin 7 and receive on some pin you're not using, like one of the bottom side pins. But SoftwareSerial is pretty inefficient. May or may not be a good idea depending on the rest of your code.
FlexIO is probably the best solution short of cutting PCB traces.