The BOM doesn’t really help, since it lists at least four different level shifters and I’m unable to tell which one is which.
I eventually figured out that what I needed to look at were the circuit board schematics. They are part of the “Jetson Xavier NX Developer Kit Carrier Board Design Package (P3509 A01)”. The part I was looking for is a TXB0108RGYR 8-Bit Bidirectional Voltage-Level Shifter with Auto Direction Sensing and +/-15-kV ESD Protect.
The TXB0108’s datasheet specifies the absolute maximum input voltage range (V_I) on the B-port as +6.5V, so I figured it was safe to just wire everything up as described in my initial question and test it out. Turns out it works! (I can read data from /dev/ttyTHS0 with 115200 bps).
So, I guess the answer to my question is: Yes. (With a caveat that doing so would exceed the TXB0108’s recommended operating conditions, but not its absolute maximum ratings. I’m not experienced enough in electrical engineering to tell if there are any additional issues I haven’t considered.)