I am trying to give a method for a future viewer of this thread, who is using 2009, an option other than it can't be done. And it would be every label style in the drawing, not every label in the drawing.
Christopher, note that we understand this. It's just that we're disgusted. Not with you or your solution, but with Civil 3D.
Compare your solution with an option to use a fixed scale, such as we have with the Point Marker Styles. With that option, we could simply specify the "Use Fixed Scale" option and key in the text height and scale we want to use, or specify the "Use Absolute Value" and key in a text height directly.
Compare that to using "IF({Drawing Scale Conversion}=600,0.1/12,IF({Drawing Scale Conversion}>1100,0.2/12,0.5/12))".
Also consider how difficult it is to even FIND the "Drawing Scale Conversion", THEN realize that you can use it in the Text Height property in the style component, THEN figure out how the "Drawing Scale Conversion" even works. I could not find anything on it in the help myself, and tried to use it several times and failed. Then, by looking at your blog, I discovered that the "Drawing Scale Conversion" is the Drawing Scale * the Paperspace Unit conversion, and was finally able to figure out how to get it to work.
The really atrocious thing is that there are A LOT of things in C3D that are just as convoluted. That's the big reason why I seriously doubt the viability of this program. If Autodesk had been improving the UI over the years, and it was constantly getting easier-to-use, then I would feel better. But the UI has basically gone unchanged since I first saw it, in C3D 2006. So for four releases now, the UI has been basically the same - an unmanageable piece of garbage. It is very disturbing.
That's why you might be detecting some emotion in this thread. It's not directed at you. It is extreme disappointment in Autodesk.