it is strange. I was looking at the pdf-file of the Mutar II. The 100-300 is even not listed there. In the Contax system broshure of 1994 it is marked as "not possible". I will try to upload the Mutar pdf file soon. I checked also the Zeiss homepage but the 100-300 is still not mentioned there. I do not know what that means. Maybe they just oversaw it ?
As long as your are able to combine it and you did not experience any problem mechanical or optical, it is fine for me...
this has just been discussed on Alexanders List recently and the answer is simple: Optically the solution is possible, with good success (At least some of the list members found the results acceptable).
HOWEVER: Mechanically the zoom only fits if extended to 300mm (and mabybe earlier) so if you push it back after mounting the back-glass from the zoom will hit the front glass of the Mutar. Probably Zeiss considers this to be to risky to officially claim it's working. But I have one Catalog, where the combination Mutar II and VS100-300 is marked as being possible, while on all others it's not.