a.l.e is totally correct, using this sort of trick is a complete hack and should only be used if you have confidence in what you're doing. (I wonder why it's not possible with 1.5 but I wouldn't be surprised if it was because it's not supposed to be done.)
Another trick might be to use an em-dash as explained here: http://forums.scribus.net/index.php/topic,2314.0.html
but, again, this is using one function of the software to do something it wasn't designed to do so should be used with caution.
Another trick, and this one is pretty horrible so I only give it by way of showing how versatile Scribus is, would be to use a table with a single cell:
* Insert a single-cell table;
* Alt-Click the table to edit the cell;
* Go to PP /Line;
* Check "Line at Top" and "Line at Bottom";
* Go to PP /Colour;
* Change the colour to whatever you want;
* Go back to PP / Line;
* Change the line width/formatting as appropriate;
* Double-Alt-Click the cell to edit the text as appropriate;
* Click the table to select it as a group;
* Go to PP / Group;
* Set Text Flow as "Bounding Box" (or whatever you prefer);
* Move the table into the text at the appropriate place (see attached).
The above technique has many pitfalls - for instance, the table has interior "margins" - so I personally would not recommend it, but someone might find it interesting.
a.l.e's idea of copy/pasting the line into the text frame is also interesting but, as already said, it has its own problems.
So that's four techniques, none of which are particularly good for what you want.
If I had to do this sort of thing I would probably just draw the lines separately - as normal lines - and then move them over the text wherever they should be. It's not a perfect solution by any means but it's probably a lot less hassle in the long run. If in doubt, just do it the low-tech way until something better comes along.