With a commit (https://github.com/makehumancommunity/m ... c9690d3de4) to master, mh will now also scan user dirs for extra materials for core assets.
For example, if an asset installation_path/data/hair/long01 is set on the toon, then when you go to the material chooser, MH will also scan the dir user_path/data/hair/long01 for additional materials.
From my cursory testing, this makes at least MargaretToigos materials for the long01 hair work when placed correctly in the appropriate path under the user dir.
I have not yet adapted the asset downloader to take this into account, but I have prepared the asset repo to accept the necessary hints. If you take a look at my example http://www.makehumancommunity.org/content/jp_blond.html, you can see that there is a new field "belongs to core asset". This is what I will use in the asset downloader to determine where the material should be placed.
Thus: if you want your third-part materials for core assets to be placed automatically (once the asset downloader has been adapted), it would be helpful if you edited your uploaded materials and set this field to an appropriate value.
Also, as a warning for future uploads: This will cause third-part materials for the same core asset to be saved in the same dir. So if two third-part materials use the same file name (for exampel "long01_diffuse_blond.png"), the second material will overwrite the first. It might make sense to take this into consideration when naming new materials. But for now, there are few enough materials that I don't think there are naming conflicts.