Yes, I think the new "rebalanced" resistances allow for a greater range of "partial" effects than the old way they did it.
I don't think 100% magic resistance means the same as a magic resistance score of 100, since (and correct me I am wrong, newb that I am) resistances can be increased past a score of 100. Rather I think 100% resistance is supposed to mean the mob is immune to any spell with a resistance check. Of course, as mentioned above, many mobs have specific weaknesses even if they are noted as being 100% magic resistant.
Of course, this really doesn't answer the initial post. If my assumption is correct that the term 100% magic resistant is NOT the same as the MR score, than I am not sure how Tash works on it.
Glad I could help to muddy the waters