This works for me, is probably not what you want, but at leat it's
consistent (it uses the interline of the main text).

\definestartstop[passus][
before={\startnarrower},
after={\stopnarrower},
style={\tfxx},
]

\starttext

\subject{It looks OK here:}
\input knuth

\startnarrower
\tfxx \input knuth
\stopnarrower

\subject{It does also here:}

\input knuth

\startpassus
\input knuth
\stoppassus


\stoptext


Andrés Conrado Montoya
http://chiquitico.org