So, I've finally received my MBP and the Dual-link-DVI to Mini DisplayPort
adapter (simply adapter from now on).
The Korean monitor has been more than worth it, it works superbly absolutely great over the digital link.
The adapter doesn't work at all if you don't plug the USB connector in also. If you plug it in, connect the display, then remove the USB plug, it will stop working instantly. So apparently that USB plug is 100% needed.
I happened to have a VGA to Mini DisplayPort
adapter lying around, so I've connected a second external display on the second Thunderbolt port. Everything works fine, my MBP now has 3 displays. I plan to replace this with a HDMI cable, since the MBP also has a HDMI port, it should work.
@Hennes commented on my question above, his answer has been very helpful.
Apparently the adapter is an active component, and it needs to be powered. It draws that power from the MBP's USB plug. To avoid losing a USB port on the MBP, the adapter provides another USB port on the other end, where you can connect whichever device was using the USB port. It seems that the adapter piggybacks on the USB port's power line, while just forwarding the others.
I wander if you could plug two of these adapters one into the other's USB port, and both in each thunderbolt port.
Anyway, the setup works great, but it's a LOT of wires to plug in.
If I were to just buy two of Apple's thunderbolt displays, I would only ever have one thunderbolt cable to connect, and the monitors would act as USB hubs too. But that's $2000 compared to $500.
For USB-C alternate modes to work, there has to be a physical connection from the alternate signal source to the corresponding pins of the USB-C port.
Unlike some older USB video adapters that actually include a small external GPU and transmit the display content as USB data, USB-C alt modes really just patch through a physical signal that the USB controller doesn't even understand. The controller is just used for negotiating what that alternate signal will be and which of the 1-4 possible connections will be used.
For discrete graphics cards, it seems unlikely that such a physical DisplayPort (or MHL) path exists from the GPU to the mainboard's USB-C controller.
Even for integrated processor GPUs, there needs to be an additional USB-C controller that negotiates the appropriate alternate mode and physically routes the signals.
The principles are explained in this presentation of the DisplayPort alternate mode.
If your current hardware doesn't support any USB-C alternate modes out of the box (or is at least prepared for it), I wouldn't expect a software update to be able to change that.
Best Answer
Apple's AV multiport adapter does work with a Thunderbolt 3 port. From this product description:
USB-C has dedicated lanes for USB 2.0 signals, so DisplayPort and USB are never mutually exclusive. One dock which does both is the HyperDrive USB Type-C Hub with Mini DisplayPort.
However, I don't believe any USB-C adapter handles DisplayPort and USB 3.1 alternate modes simultaneously. If you want both you need to go full native Thunderbolt 3.