[pdftex] Howto activate overprint (/knockout)

[JEBs]

I'm developing a PDF document which shall be printed by a professional 
printing company.

The whole document has a light-yellow background so that I need to 
activate (most times, but not always) the "black overprint" option as 
well known/available in PS or PDF.

First I though it should be definable on color level but while searching 
the whole web (and this ML) I couldn't find anything related which 
helped me except PDF native codes (/op /OP /OPM ...).

Now I'm sure that I'm on the right way but guess that I do something 
stupidly wrong.

The way I tried to get it working is:

----- snip ----- snip ----- snip ----- snip
...
\pagecolor....

\pdfobj{<<
/op true
/OPM 1
 >>}
\edef\opObj{\the\pdflastobj}
...
...
\color{black}
{\huge{\textbf{\pdfrefobj\opObj Test 5 6 7}}}
...
----- snip ----- snip ----- snip ----- snip

but it don't work. If I look into the PDF I can see the created object 
as i.e. "8 0 obj ..." but it's never used ("8 0 R").
However I figured a lot with all the stuff like "\pdfliteral{<< /Type 
/ExtGState /op true /OPM 1 >>}" too, but never got a working result :(.


Does anybody has an idea or knows a way ?

Thanks in regards
    JEBs