How can I configure the UART to have an Open Drain (OUT_OD) TX pin?
You can’t; this isn’t a supported feature of the underlying silicon. The easiest way to do this, if you just want a 3v3 open drain UART, is by putting a schottky diode in line with the TX, cathode facing the imp.
This will cause the line to get pulled down to ~0.4v when the UART is driving low, but won’t do anything to the line when the UART is high (idle).