A mainboard with such a feature just needs a hidden remotely accessible subsystem that is completely protected by TPM and which has access to your system through DMA, to read bytes and to write bytes in whatever system (RAM, Harddisks, SDcards, USB, WLAN, etc).
Consider it a "dual system" mainboard with the usual hardware where you have full control, plus a controlling subsystem where you have no access at all. Everything could be done remotely but you couldn't see any evidence.
I agree that this could be done in principle, and even that it isn't unlikely that people are thinking about doing it. But is there any evidence that such a subsystem has ever been created? Are there any additional technical details surrounding this?