Version 0.3 of IceBoot disconnects the ARM from the ch340 device once the firmware has booted, allowing it to be used as a USB UART by the FPGA.
We use this our serial port in the OPC6 system project.
Here's the .pcf file we use:
And here's the top level design:
Feel free to make use of the "one page" UART:
There's likely much better implementation out there...