I couldn't figure out how to access the UV channels via shader. Is there any info available already?

I think it all depends if Cinema4D allows to have multiple UVs in the same mesh and assign different shaders to them. Using MeshFromStrands we can add multiple channels to the hair mesh, but I can´t find a way to visualize them in C4D. This would be the first step. 

Jeordanis Figuereo (Product Designer. EPHERE Inc.)

I just found out that when creating multiple UV sets in MeshFromStrands using the Map Channel Count parameter the tags for each UV set are created and cached, but are invisible. Just have to figure out how to connect each tag to a different texture. 

You can have 2 uv per mesh by duplicating the uv tag the order in OM goes from left to right. So the texture tag on the right of the second uv tag will be assign to that and any on the left of it will be on the first uv tag. Tested it though doesn't seem to be working though. 

What is OM? Object Manager? Something like this? 

Jeordanis Figuereo (Product Designer. EPHERE Inc.)

Thanks for the link. When using this method, you expect both materials to be rendered at the same time? I tried this method with a polygon plane and it works, but only one material is rendered at a time, unless I´m doing something wrong. How do you blend them? Like your first example: One material for the feather mask and one material for the color. 

I´m already working with one of the devs to make this work properly. This conversation is being useful to understand how users can use this feature. 

Jeordanis Figuereo (Product Designer. EPHERE Inc.)

You guys could implement a shader that has direct access to a linked Hair object. Then, to retrieve them, get the cache and extract the UVs.

Ideally this should work without additional plugins or shaders that would need maintenance on every render engine probably.

I made it work after some modifications to the plugin made by Roger. Now MeshFromStrands gives specific names to the generated UVW tags: channel1, channel2, channel3, etc, depending on how many channels you created on MeshFromStrands. Now that we now the names, we can call the them in the texture node. As you can see in the following image, one texture is used to drive the opacity and another for the overall color:

In this case I tested with Redshift, which allows to specify a UV channel for each texture node. I created two channels in MeshFromStrands. One with Per-Strands UVs for the opacity texture and another with the distribution mesh UVs for the overall color. In the Redshift texture node for the opacity texture I specified channel1 in the UV section and channel2 for the overall color texture node. This is how it looks: 

For this to work you need the latest Ox nightly build available here: https://ephere.com/plugins/maxon/c4d/ornatrix/downloads.html

Jeordanis Figuereo (Product Designer. EPHERE Inc.)

Try with index 0 and 1 instead. 

Jeordanis Figuereo (Product Designer. EPHERE Inc.)