Is it possible to use regular brown chocolate icing under white fondant? Or will the brown show through the fondant?

absolutely, it will not show through

I just asked this same question and the response I got was to roll it a little thicker. I tried it and it was ok once I covered w decorations.

It won't show. The white wedding cake in my pics was iced with chocolate ganache and covered in white fondant...no problem at all!

I have made MMF and while it didnt show through 100% the colour was a little muddier than I'd hoped. So my recommendation is to roll it thicker