I will write about this more later in the context of some stuff I saw at Pitti, but an important consideration for a dinner suit is how much contrast you want between the facings and the rest of the jacket. Obviously a mohair has some sheen to it, so the lapels don't stand out as much. On the other hand, all of you is a little more shiny. A more matte fabric will highlight the facings more.
I wouldn't worry about formality too much in the context of a dinner suit. If you've got the facings and are wearing the black bow tie, people get it. Black tie is not meant to be a very restrictive dress code, and is meant to have a relaxing-with-friends air to it rather than a "formal" one IMHO.
IIRC you can get different silks for facings. Matte or Satin. I don't think there's a problem in that regard.
Again, Harrisons have a Mohair Barathea in their Evolution book. That's perfect DJ material.
Traditional Black Tie is by definition a restrictive dress code and formal. There is room for manoeuvre but not much as I said in another post.