On real sites, though, it's important to keep control over exactly when the increases in scale are triggered.

I flatly disagree with this. It is typical for some significant alterations of layout to take place somewhere in scaling it down, but that is not incompatible with something like this. It is entirely feasible to have most of your responsive design using a technique like this but at a certain point change the layout; also, the font-size need not be a globally continuous function; it's entirely reasonable to have local discontinuities where you adjust the m or c of font-size = m x + c for a range of widths.