\defineblock[myframes]

\unexpanded\def\GetDim#1#2{
\setbox\scratchbox\hbox\bgroup\useblocks[#1][#2]\egroup%
\directlua{%
local n = tex.getbox('scratchbox')
context("\\definemeasure[#2W][" .. (n.width /65536)  .. "pt]")
context("\\definemeasure[#2H][" .. (n.height /65536)  .. "pt]")
}}


%------------------------------------------------------------------------------

\starttext

\beginmyframes[FTitle]
\framed
  [align=normal]
  {initial framed for a title zone\\
   early in the source code but not yet typeset\\
   with a long sentence to take place horizontaly and for demonstration}
\endmyframes

\GetDim{myframes}{FTitle}

\framed
  [align=normal,
   width=\measure{FTitleW},
   height=\measure{FTitleH}]
  {a second framed, with dimensions equal to the original frame, typeset before the original}

\useblocks[myframes][FTitle]

\stoptext

%------------------------------------------------------------------------------