I have a fork of the firmware for the CANPico boards and have added a second USB serial port for use by MIN (a reliable serial transport protocol) to send and receive remote control commands from a host (in my case, pushing CAN frames received to a host to see in Wireshark). It's a lot more robust than trying to multiplex the REPL serial port into raw mode.

I hardwired the second port by bumping CFG_TUD_CDC to 2 in tusb_config.h and the table usbd_desc_cfg in tusb_port.c.