Usually when I think of mini-series, I think of "The Thorn Birds" or "Shogun" and the like that is basically one very long movie, divided out into parts to fit a TV show. Maybe something like you described would work better, but I still think that the incomplete nature of the Simarillion and the plodding story would make for some good episodes, and some episodes that would be almost impossible to watch. Unless one wanted to do some massive re-writing job. They did some changes for LOTR, but the Simarillion would need much more.