Are there any USB NKRO keyboards on the market yet?

[Soarer]

It would - that's what nearly all keyboards with multimedia keys will do. But it's still only a 6KRO keyboard; the second endpoint doesn't contain more 'regular' keys (and if it did, it wouldn't be compatible with Macs).

BIOS only looks for endpoints that are flagged as being a 'boot keyboard', possibly issues a set protocol on them, and uses the predefined 6KRO layout.

OS resets the device, reads descriptors for all endpoints, doesn't issue set protocol, and uses the descriptor-defined layout for each endpoint.

[Kamen Rider Blade]

So even though full nKRO over USB is possible, assuming you're using Full Speed USB and probably a 16 byte report with each key being a bitmap in that report, what about the hardware level?

Aren't most keyboard hardware not designed properly to have full NKRO?

I remember reading some boards needing diodes on each circuit to do full NKRO which gets expensive.

Normal Matrix keyboards only have Horizontal Rows & Diagonal Columns (2D array) for registering keys.

Is there another hardware design that allows full NKRO?

If it's a Keyboard Matrix, I would assume having a 3D matrix would make it possible (Horizontal Row & "\ Left Slanted Diagonal" & "/ Right Slanted Diagonal") to detect every key combination input out there.