Agree with bwhite for the most part. BIS miles as a tiebreaker doesn't make all that much of a difference once you've already broken it down by status and fare. When you think about it, there are already dozens of levels of differentiation, even ignoring for a moment the Reserve card:

DM on Y
PM on Y
GM on Y
FO on Y
DM on B
DM on M
DM on H
...

All you're proposing doing is splitting each one of those into two, which doesn't really reshuffle the order too much.

You'd have to put it as a tiebreaker before fare class, and that I would be opposed to. Within status levels, those who are CURRENTLY flying on an expensive fare should get the upgrade of someone who has flown more but is on a cheap fare.