Many thanks, Wolfgang, for your quick and complete solution to my riddle.
As Hans has commented, it is really brilliant.

I must confess that I do not understand yet the substance of it, i.e. the
definition itself of *Fleuron*, but I will keep studying it and will end
for getting it, I hope.

For the moment I have analyzed and elaborated a bit on your example, first
and mostly by commenting it, but also by slightly modifying it so that it
shows more clearly the characteristics of your solution. I attach it here
just in case it could be as helpful for someone else as it has been for me.

Many thanks again and good luck!

Francisco

% Fleuron.tex

% Example script of the creation and use of a documental ornament


% We first define the documental ornament

% under the traditional name of *Fleuron*

% in the way brilliantly formulated by Herr Schuster

\def\Fleuron

{\par

\leaders\vbox to \lineheight{I am the \em Fleuron\em}\vfil}


% and create a page 10 lines high for testing its behaviour

\setuplayout[lines=10]


% We put *Context* to work

\starttext


% Method 1 (really?)


% Typesetting the ornament as the first element of the page

% results disappointing: it just does not show up

\Fleuron


% but other texts that follow it get their normal treatment;

% for instance, this sentence

I am not really the start:\par

\ \ \ the \em Fleuron\em\ went before me\par

% or this automatically generated text

\dorecurse{4}{Line \recurselevel\par}


% Even the *Fleuron* is treated the same,

% if it is the last element of this page

\Fleuron


% so we create a new page and repeat the same

% instructions

\page

I am not really the start:\par

\ \ \ the \em Fleuron\em\ went before me\par

\dorecurse{4}{Line \recurselevel\par}

\Fleuron

I am after the \em Fleuron\em\

% surprisingly, although there is room available,

% the ornament disappears

% if there is more text coming after it


% so we go to another page, the third one,

% for our last and definitive test

\page


% This time the space of the page is full

\dorecurse{10}{Line \recurselevel\par}

% when the turn of the *Fleuron* arrives

\Fleuron


% The result is that not only the *Fleuron* does not show up

% in this page, which was to be expected because it is full,

% but even the implied next page, the fourth,

% does not get generated by *Context*. PERFECT!



\stoptext