What is the meaning of the "-osic" suffix on a metal (e.g., cobaltosic oxide)?
In the course of searching for examples of usage of the "-osic" suffix, I think I found the answer to my own question.  From the last link in the question, a web store selling pigments (emphasis added):


  The source of natural black oxide usually is from an iron ore called magnetite. Magnetite, also known as lodestone, is a ferrous ferric oxide (ferrosic oxide) that is a heavy, black color and opaque. 


So, it would appear that the suffix indicates a compound of mixed oxidation state: a combination of -ous and -ic forms of the metal.