Just before going to bed, I've found a solution with all your help and the help of Taco Hoekwater through the ntg mail archive :

\def\startSlide%
{\def\stopSlide{\checkSlide\doif{\getvariable{temp}{publish}}{yes}{\page\getbuffer[Slide]}}%
\dostartbuffer[Slide][startSlide][stopSlide]}

Let me know if it's my philosopher's stone...
I've tried it and it works so far.

No more ugly gobbling, my mind is free.

So depressing problem and so simple and elegant solution...

On 07 Nov 2014, at 16:17, Hans Hagen <pragma@wxs.nl> wrote:

> On 11/7/2014 2:26 PM, Pierre-François Bonnefoi wrote:
>> Hello Hans,
>> 
>> thank you for contributing to solve my problem.
>> 
>> I've tried your proposition : how can I get the content of the buffer
>> afterward ?
>> I've tried \getbuffer[Slide] with no success on MKIV beta that I've just
>> updated.
> 
> \starttext
> 
> \definebuffer[Slide]
> 
> foo
> 
> \startSlide
> bla bla bla
> \starttyping
> language C code
> \stoptyping
> bla bla bla
> \stopSlide
> 
> bar
> 
> \getbuffer[\thedefinedbuffer{Slide}]
> 
> \getSlide
> 
> \stoptext
> 
> 
>> Does a buffer work only for verbatim content ?
> 
> no, for everything
> 
>> best regards,
>> Pierre-François.
>> 
>> On 07 Nov 2014, at 12:38, Hans Hagen <pragma@wxs.nl
>> <mailto:pragma@wxs.nl>> wrote:
>> 
>>> 
>>> \starttext
>>> 
>>> \definebuffer[Slide]
>>> 
>>> foo
>>> 
>>> % \skipSlide
>>> \startSlide
>>> bla bla bla
>>> \startyping
>>> language C code
>>> \stoptyping
>>> bla bla bla
>>> \stopSlide
>>> 
>>> bar
>>> 
>>> \stoptext
>>> 
>>> 
>> 
>> --
>> Bonnefoi Pierre-Francois                |    E-mail : bonnefoi@unilim.fr
>> <mailto:bonnefoi@unilim.fr> http://libpfb.so/
>> Universite de Limoges, Laboratoire XLIM |       Tel : 06 28 18 03 38
>> 123 av Albert Thomas                    |  Mrs. Peel, we're needed...
>> 87060 Limoges CEDEX - FRANCE            |                 The Avengers.
>> 
>> 
>> 
>> ___________________________________________________________________________________
>> If your question is of interest to others as well, please add an entry to the Wiki!
>> 
>> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
>> webpage  : http://www.pragma-ade.nl / http://tex.aanhet.net
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>> wiki     : http://contextgarden.net
>> ___________________________________________________________________________________
>> 
> 
> 
> -- 
> 
> -----------------------------------------------------------------
>                                          Hans Hagen | PRAGMA ADE
>              Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
>    tel: 038 477 53 69 | voip: 087 875 68 74 | www.pragma-ade.com
>                                             | www.pragma-pod.nl
> -----------------------------------------------------------------
> ___________________________________________________________________________________
> If your question is of interest to others as well, please add an entry to the Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://tex.aanhet.net
> archive  : http://foundry.supelec.fr/projects/contextrev/
> wiki     : http://contextgarden.net
> ___________________________________________________________________________________

-- 
Bonnefoi Pierre-Francois                |    E-mail : bonnefoi@unilim.fr http://libpfb.so/
Universite de Limoges, Laboratoire XLIM |       Tel : 06 28 18 03 38
123 av Albert Thomas                    |  Mrs. Peel, we're needed...
87060 Limoges CEDEX - FRANCE            |                 The Avengers.