\definebar[mybar][inlined][offset=.25ex,color=red]

\starttext

    \startformula
                a      \alignhere= b + c \breakhere
        \mybar {d + e} \alignhere= f     \breakhere
                g      \alignhere= h + i
    \stopformula

    \page

    \samplefile{knuth}
    \startformula
        \text{something} + \text{really} + \text{really} + \text{long} \alignhere= \sqrt{x} \numberhere
        \texthere[inbetween]{\input{knuth}}
        d \alignhere= e + f \numberhere \breakhere
        g \alignhere= h + i \breakhere
        j \alignhere= k + l
    \stopformula
    \samplefile{knuth}

\stoptext