Extension tubes work fine. I have a set of the Kenko version, quite a bit cheaper than the Canon original. Use those on a very regular basis. I hope Zeiss will bring back a true 1:1 macro. I talked with the Zeiss rep at the PhotoCon Los Angeles this May, and she said that this is one of the most frequent requests they get. I hope they listen.
1.4x III does NOT work on the MP100 Classic (have not tried it on the Milvus), because of some mechanical interference. Don't know exactly what is doing it, but it's not working by a hair. I suspect that the Milvus has the same backend as the classic, so I'd expect the same result.
1.4x does work on MPE65 and gets you to 7:1 on FF. If memory serves me well, the 1.4x II did not work either. I have not tried other TCs on the MP100.
As a long macro, I eventually got the Canon 180M, despite all its flaws. For long distance close-ups (rattlesnake portraits in the wild), I use the 300/2.8 IS with extension tubes.
Have fun with your new lens!