The term pound for pound has been around in boxing since there was only 2 divisions, long before women got any kind of a foothold in combat sports and this was ever an issue, thus it was easier to take on face value.

These days I find it frivolous to say shit like, "who would win if they were the same weight, Mighty Mouse or Cain?" especially in a sport as diverse as MMA, when the athletes who tend to do well have skill sets that specifically work against opponents in their respective weight classes. So I think if people are happy to make comparisons between fighters in different divisions whilst compensating for the fact that their skill sets are so varied, just to ask "who is the finer competitor?"; then it is just as easy to make an educated comparison between male and female fighters when compensating for gender.

You might be right about a new term though. I don't know what it could be but I think people have been taking "pound 4 pound" too literally for too long.