Are there multi-filament hardened nozzles or equivalent systems?
Yes, but there is no hardened version of the "Diamond" nozzle design. Basically when you want to use a hardened nozzle, or a Ruby nozzle in combination with color mixing (I specifically refer to mixing solutions as the melting chamber is smaller than filament changing solutions and your request for none or minimal purging towers) you need to order a design that accommodates the replacement of the nozzle. In such a case you can remove the standard nozzle to replace it for a hardened version.

There are a few 2 and 3 filament input designs that are offered on typical auction and Chinese selling sites. Below is an example for the Zonestar M3 mixing color hotend with a replaceable nozzle.