I understand.
Using the full program
ebook-convert when we just wish to expand the CSS is like using a hammer to kill a fly. We are miles away from your previous "micro-surgery" operation.
If you allow me to do it, I'd like to address Kovid Goyal about it. He alone, knows which part of the Calibre code is used specifically to expand a CSS style-sheet, and, who knows?, he would be so kind as to point to it? It can't be that huge. Once a stylesheet is taken out of the water, it seems a not so different job to expand it or to convert some of its values. and both could be done successively.
On the other hand, he could see a benefit to this as he could add the cm2em conversion feature to ebook-convert, which strangely, does not have it yet.
EDIT: when posting, I realized this was nearly done!! Thank you Kovid Goyal !!