Any nFET with on-resistance in the 0 to 200 ohms should be quite happy to interface to logic - assuming using a
2k2 pull-up or so. Clearly something cheaper than a large power MOSFET would be good! Without knowing more
I don't see a reason why a bipolar NPN transistor can't serve the same role (remember base resistor). Two NPNs
could be used, the first as an inverter...
A 74HC244 could also be used with the UART's TX going to one of the output enables, and LOW to a buffer's input -
then the buffer's output will work like open-drain. For instance pin1 form TX, pin2 LOW, pin 18 to RX and the pull-up
resistor. This works because the output enables are active low, so no inverter needed.