[lltx] linebreak_filter problems

[Frank Mittelbach]

Hi folks,

can somebody help me to identify if I do something wrong or if there is 
a bug with linebreak_filter.

Th problem is documented at length in

http://tex.stackexchange.com/questions/59176/lastnodetype-not-working-as-expected-in-luatex

In a nutshell I try to replace (at certain times) TeX's linebreaking 
algorithm and replace it with a simple version that just packs up the 
hlist into an hbox and returns that.

If I do that the resulting box appears on the current vertical list (ie 
is seen via \showlists) but at the same time

  \lastnotetype  returns -1
  \lastbox       picks up a void box

ie to some parts of TeX the list appears to be empty. Here is an MWE

\def\partest{%
    \directlua{%
      function hpackparagraph  (head)
        return node.hpack(head)
      end
      callback.register("linebreak_filter", hpackparagraph)
   }%
   \par
}

\setbox0\vbox{ABC\par
 
{\tracingonline1\showboxbreadth\maxdimen\showboxdepth\maxdimen\showlists}%
   \showthe\lastnodetype
}

\setbox0\vbox{ABC\partest
 
{\tracingonline1\showboxbreadth\maxdimen\showboxdepth\maxdimen\showlists}%
   \showthe\lastnodetype
}


Any ideas what goes wrong or how I should alter things to make this 
possible?

thanks
frank