The Morosi Campaign was part of the 9th Frontier or 9th Kaffir War and is mentioned in "The Washing of the Spears" by Donald R. Morris. I can't recall how much was said about it since the book concentrates on the Zulu War. It is mentioned because what instigated it was Chelmsfords' invasion of Basutoland. Books on the 9th Kaffir War or colonial history of Basutoland would have more on the subject. Also since Herschel was a district in the Cape Frontier, a history of that region may make mention of the Herschel Native Contingent. I would also try sending a written (snail mail) letter of inquiry to the South African Military Historical Society. I'll bet they could be of great help with your quest.
I pulled these recommendations off the site I mentioned in another thread (Regiments.org):
9th Kaffir War:
1877-1878 Ninth Kaffir War 1877-78, by Ralph Zuljan (OnWar.Com)
Jahreschronik 1870-1899, by Bernd Marek (Chronologische Kriegsgeschichte)
Research Collections and Guides:
British Army: Campaign Records, 1816-1913 (TNA Research Guides: Military Records Information 25)
Have a look here, Mark, for any information on your medal: http://www.regiments.org/wars/19thcent/ ... tm#museums
(click on "Battles & Honours")
I'm afraid I don't know much about medals so I hope this list is of some help.