I think I get your idea and such an application would be nice but I'm afraid, that's not how these things work. The CMA18p is just an audio device with output plus recording/input channels to the USB-host device (normally your PC, Mac, Phone, DAP or tablet ...) - just like any other audio interface for studio recording etc.
It is not in itself a USB-host for other devices and not capable to stream USB-audio to an external DAC or to "negotiate" any power delivery with an attached dongle or self-powered DAC.
Thus plugging a DAC/Dongle into the USB-C-port of the CMA18p while in BAL/Line-In recording mode does not provide any useful USB-audio data for an external DAC to play. The CMA18p is a kind of passive device, controlled by the USB-host which provides or retrieves the USB-audio data to/from the CMA18p.
Despite knowing all this, I tested it anyway (I am a scientist so I experiment) to check how an external DAC and the CMA18p behave in this setting and interestingly, the REC- and XMOS-USB-Processing LEDs of the CMA18p lit up but the external DAC are not recognized and not powered ... as expected.