btex etex trouble

["Eckhart Guthöhrlein" <eckhart_guthoehrlein@public.uni-hamburg.de>]

I have some questions/remarks on inclusion of mp graphics with btex/etex. 
Sorry if I overlooked something in the docs, I'm rather exhausted now...

Is there a way to tell context to write mpgraph.mp files suitable for plain 
tex, not for context? I got the following error message

This is TeX, Version 3.14159 (MiKTeX 2.1)
(D:/localtexmf/mpx52.tmp/mpx314.tex
! Undefined control sequence.
l.2 ...bal \loadfontdefinitionfileoncetrue
                                           %
! You can't use a prefix with `\shipout'.
<to be read again>
                    \shipout
l.3 \shipout
             \hbox{\smash{\hbox{\hbox{% line 84 C:\DOKUME~1\Eckhart\De...
[1] )
(see the transcript file for additional information)
Output written on mpx314.dvi (1 page, 200 bytes).
Transcript written on mpx314.log.

 >> mpgraph.mp
 >> mpgraph.mpx
! Unable to make mpx file.
l.84 draw btex
                Argh etex
Transcript written on mpgraph.log.

by tex when called by mp. I managed to instruct mp to call context instead 
of plain tex, so it works now, but plain tex would be enough for me (simple 
text, cm fonts) and considerably faster. Inspection of mpgraph.mp showed

graphictextformat := "context" ;
graphictextdirective "\global \loadfontdefinitionfileoncetrue " ;

Can this be changed? Or something else? Or is it impossible / undesirable 
to use plain tex for some reasons I don't see?
Furthermore, the result of running mptotex manually looks somewhat strange 
to me:

% line 82 mpgraph.mp
\global \loadfontdefinitionfileoncetrue%
\shipout\hbox{\smash{\hbox{\hbox{% line 85 mpgraph.mp
Interpretation}\vrule width1sp}}}
\shipout\hbox{\smash{\hbox{\hbox{% line 87 mpgraph.mp
Verfeinerung}\vrule width1sp}}}
\end{document}

It can be compiled using context and further processed by dvitomp, no 
problem, but where does \end{document} come from? Looks like latex.

The trouble is already solved, but I would be grateful for some clarifying 
remarks.

Eckhart