I don't know if it helps, but if the USB has any type of memory space, or some sort of output buffer that it uses to send data, then you can practically place your message there using a DMA transfer.
Not a pro at USB functionnalities, so I'm not sure if it'll work.
Thank you, Dana.
AN57473 doesn't mention DMA at all, but AN56377 has some details I was unaware of. I need to experiment with this a little more.
Do you know where these odd DMA sources ("USB_EP17_DMA_Done_SR" and "USB_EP8_DMA_Done_SR") are coming from? As I mentioned, I have only one endpoint configured in the USBFS component, and that is EP1 as an IN endpoint.