@AndrewStacey: I did some basic tests with the highlight library. Here is the code I used:

\documentclass[a4paper]{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\let\usetikzmarklibrary\usetikzmarkextra
\usetikzmarklibrary{highlight}

\newcommand{\tikzmarkhighlightset}[1]{
	\pgfkeys{/tikz/highlighter/.cd,#1}
}

\newcommand{\tikzmarkfadeset}[1]{
	\pgfkeys{/tikz/fader/.cd,#1}
}

\begin{document}
%\[\highlight[colour=green!50]{x+y}=100\] % doesn't work?
\tikzset{/tikz/highlighter/.cd, initial indent=-1pt,final extent=1pt,colour=green!50}
\[\highlight{x+y}=100\]

I added some commands and aliases to keep the code clean and there are some comments about things not understood, for example: how vhighlight or \vlstart and \vlend gave to me strange results, or why I could customized the width of the highlighted part but not its height.