Not sure if there are side effects but this works (requires mkiv and a recent installation):
\setuplayout[grid=yes]\showgrid \setupbodyfont[14pt]
\usemodule[annotation]
\define[2]\ZitatCommand {\topskip\dimexpr\topskip+\lineheight/2\relax \startnarrower “#2”% \stopnarrower}
\defineannotation [zitat] [alternative=command, command=\ZitatCommand, spacebefore=halfline, spaceafter=halfline]
\starttext
\startzitat \input tufte \stopzitat
\dorecurse{2}{\input tufte\par}
\startzitat \input tufte \stopzitat
\dorecurse{3}{\input tufte\par}
\startzitat \input tufte \stopzitat
\input tufte
\stoptext
Indeed, this solves the grid issue for block quotations broken across pages. Many thanks! Unfortunately introducing a "\blank" just before the last "\input tufte" in the above example breaks grid alignment. Oliver