Hi, David,
The LPC55xx supports CDC class as device as you expected, in CDC class, it has two channel based on CDC class protocol, one channel is controlling channel, with which the host side can set up baud rate, data format, data width..., another channel is data channel, with which data can be transferred.
In SDK package, we have implemented the usb stack including CDC, and we have developed a lot of examples.
https://mcuxpresso.nxp.com/en/welcome
With the CDC class, you can transmit and receive data via USB, with the data, you can transfer them via SPI, uart, I2C, I think it is so-called bridge.
Hope it can help you
BR
XiangJun Rong