Yes, pinMode(pin, INPUT_PULLUP) works great on Teensy-LC. No need for a real resistor.
Does this hold true for the Teensy LC's D0 (which I'm using as a serial receive RX1)?
Yes, all the digital pins can do internal pullup or pulldown.
If you are using the Serial interface(s) the function Serial.Begin() will override the pin direction though. No need to specify pin direction with serial.