> It should be easy to to do with javascript, like Free nodelet hack: Auto-Fold threads in RAT for special browsers

Thanks I already forgot I published this code ... =)

Anyway it's even easier, nodes with replies are marked with CSS class nnt-line-haskids

This setting in RAT Style Settings

 
.nnt-line-haskids {
  display:none;
}
[download]

will hide them.

Currently only "[Perl6] Problem using Inline::Perl5" will be left shown then.

This could easily be interactively toggled with JS or used for other effects, like changing coloring ... and so on ...

Further implementation left as an exercise for the interested reader...

... TIMTOWTDI and YMMV! :)

As a poor mans alternative ...

The page settings of RAT allow "Show toggle on the left".

Since Nodes without replies have no toggles, they won't be intended and stick out.