I made it:

\define[2]\epigraph{
  \blank
  \startframedtext[middle][frame=off,offset=none,width=fit]%
  \startalignment[middle]
  \startcolor[darkgreen]#2\stopcolor
  \stopalignment
  \startalignment[flushright]
  {\it #1}%
  \stopalignment
  \stopframedtext
  \blank
}

I think it does not look bad. But I have to let it grow on me.

\define[2]\epigraph

  {\blank

   \startframedtext[middle][frame=off,offset=none,width=fit,align=middle]%

   \color[darkgreen]{#2}\endgraf

   \rightaligned{\it #1}%

   \stopframedtext

   \blank}