Hi,
As I told you I have never used the UART with HW control flow and I have not seen examples in FW however the part that concerns the setting of the UART does not differs much from the example that David has included among the examples of FW.
I had found chapter 21 of the datasheet
http://ww1.microchip.com/downloads/en/DeviceDoc/61107G.pdf for PIC32MX.
I found the registers section 21-1 in particular the UEN<1:0 > that enable CTS and RTS.
Just PPS and FW32 can connects D0/D3 as you expect in your shield (i.e. the CTS on FW-D2 and the RTS on FW-D3).
But on the USB you will have the communication flow between Zulu and FW (i.e. D0 and D1)
At this point if you want to use FW console.write() you have to use UART2 (among the examples I seem to remember that there is an example for the UART2) and divert it by PPS to other FW pins .
You should definitely do the loopback tests before you connect something.
Keep an eye on the registers that enable IrDA because you definitely have to disable it.
I'm sorry I can't help you anymore.. however some more LEGO bricks.