\unprotect
\newbox\hightlight_tmp_box

\definemathframed[_highlight][
    location=mathematics,
    frame=off,
    background=color,
    backgroundcolor=lightred,
    backgroundoffset=0.25ex,
]

\tolerant\def\highlighthere#1\alignhere#2#3\breakhere{%
    \setbox\hightlight_tmp_box=\hbox{%
        \m{%
            \forcedisplaymath%
            #1#2\null%
        }%
    }%
    \alignhere%
    \phantom{%
        \null#2\null%
    }%
    \mskip -8mu%
    \hskip -\wd\hightlight_tmp_box%
    \lower 1ex \hbox{%
        \_highlight[
            loffset=\wd\hightlight_tmp_box,
            location=keep
        ]{%
            \hskip -\wd\hightlight_tmp_box%
            \forcedisplaymath%
            #1#2#3%
        }%
    }%
    \breakhere%
}
\protect

\starttext
    \startformula
        a \alignhere= b + c \breakhere
        \highlighthere d + e \alignhere= f \breakhere
        g \alignhere= h + i
    \stopformula
\stoptext
