documentclass{article}
setlengthparindent{0pt}
begin{document}
  defhdr{1b)}
  defmark{%
    strut{%
      vadjust{%
        vbox to 0pt{%
            kern-dpstrutboxrelax%
            smash{llap{hdr}}%
            vss%
          }%
      }%
    }%
  }
   mark Once upon a different time\%<- Works nicely

   mark Once upon an equation ( displaystylesum_{i=1}^{infty} )%<- Unaligned
end{document}

(displaysylesum_{a}^{b})

newboxppmarkbox
newlengthmarkskip
setlengthmarkskip{4baselineskip}

setboxppmarkboxhbox{%
vrule@height.7markskip
        @depth.3markskip%
        @widthz@}%
defmarkstrut{unhcopyppmarkbox}