While there are certainly customers out there who think along these lines,
most of the enterprise customers I've run across in the past who would be
in the market for data center colo would just as soon play the how-many-
servers-can-i-jam-into-this-rack game, which is one part of the
how-many-racks-can-i-jam-into-this-cage game for some folks...

You might get some traction with the responsible deployment angle, but I
could only guess at how much traction...

Speaking as one who used to play both of those games, it's a hard habit to break. The folks paying the bills don't like to see empty space, because they translate that into wasted $$'s. It's especially difficult when trying to justify building out an additional cage (or making the one you have bigger if there's empty adjacent space) because your current one is at max kva per ft^2 - but has physical room for several more racks. The trick for us was getting enough management clue in place to where you (gasp!) plan ahead for your power needs first and make raw ft^2 the secondary concern.