[luatex] inserting elements after line break

[Arno Trautmann]

Hi all,

I thought this would be a quite easy task with luaTeX, but I didn't
manage to do it:
I want to insert something (say, a rule) after a paragraph has been
broken. So my first attempt was to register a function to
post_linebreak_filter, but if I try to tex.print() something in that
function it just doesn't do anything. (writing to console works, so I'm
sure it is registered and is called)
So the question is: Is it possible to insert anything printable right
after a paragraph has been broken? If so, how? That material should have
no width and height; it should not affect the text at all.
(In the end, I want to draw a fine rule under each line in that paragraph.)

Thankful for every hint,
Arno

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