The concept of "bond strength" in ambiguous. See
http://en.wikipedia.org/wiki/Bond_strength. As you can see, indeed the Mulliken overlap population is one type of measure of the bond strength, but there are others, and each one is computed in a different way. As the Wikipedia page says, to calculate the bond energy "requires lengthy calculations, even for the simplest bonds", and similar is true for bond-dissociation energy. Plus, it's mostly a concept used for molecules where the system makes sense if you break a single bond.
So, for a complex system, probably the overlap population is as good as you can get it. As with most things - the important thing is to define and motivate why you quote and use a particular number in a publication, and not just rely on any standard definition of ambiguous concepts. So, don't call it "Bond Strength", just call it Mulliken overlap population, and say that you use this as a measure of the bond strength.
