@c.man It should work.
The UniPi board is adding drivers to the existing Raspberry Pi UART signals.
CODESYS for UniPi doesn't add anything to support the serial ports.
CODESYS for Raspberry Pi supports the raspberry Pi serial ports and Modbus, including the ASCII mode.
If you require further assistance on the software side of this I suggest that you use the CODESYS raspberry Pi Forum.
If you have signalling issues then I'm sure UniPi can assist you.