\startuniqueMPgraphic{Label}
path p; p := (0,0) -- (OverlayWidth,0) -- (OverlayWidth, OverlayHeight) -- (0, OverlayHeight) -- (0,0);
draw p withpen pencircle scaled 1pt dashed withdots;
setbounds currentpicture to boundingbox OverlayBox;
\stopuniqueMPgraphic

\defineoverlay[Label][\useMPgraphic{Label}]

\def\dottext#1%
{  \mframed[frame=off, background=Label, location=low]
  { #1 }
 }

\setupcolors[state=start]

\def\rndgraymath{\mframed[frame=off,	% for math have to use \mframed
    corner=0, background=color,  backgroundcolor=gray,  backgroundoffset=3pt] }

\starttext

Large font with rounded gray background 
\startformula
        \rndgraymath{\tfc e^{i\pi} + 1 = 0 }
\stopformula \blank[big]

Large font in dotted rounded frame & gray background (should be)
\startformula
        \rndgraymath{\dottext{\tfc e^{i\pi} + 1 = 0 }}
\stopformula \blank[big]

Large font in dotted rounded frame & gray background  (should be)
\startformula
        \dottext{\rndgraymath{\tfc e^{i\pi} + 1 = 0 }}
\stopformula

\stoptext