How substitutable is Statistics for Mathematics in graduate admissions?
This can only be answered by the university in question, probably at the level of the math department. It might depend on the specific statistics courses that you want math credit for. As you say, statistics overlaps with math a fair amount and many of the courses depend on a mathematical outlook and way of thinking. 

In general, though, I think you are more likely to have them accepted than rejected, but you still have to face the fact that there will be a lot of other applicants for whom no exceptions need be made. So in a competitive place it might be the competition with those having a more standard background that will work against you. 

But you have to ask them, and, as usual, you need to make your own case for why you will be a success. The evaluation depends on many things not just a bit of bookkeeping.