Sorry, I've no idea about RTL ;/
And yes, support for Qt3 was dropped in Qwt 6.x.x so anything before that should work just fine.

I know there's a plugin for the designer, but most time I've heard about it was mentioned in context of QtCreator on windows.
I would imagine you can use it with QtDesigner alone but you'll have to try it yourself - I personally don't use it

Here's few links that may help you, they are all for windows but you may get an idea what needs to be done:
http://doc.qt.nokia.com/qtcreator-2....g-plugins.html
http://qwt.sourceforge.net/qwtinstall.html
http://stackoverflow.com/questions/1...gin-for-qt-4-5
http://www.qtforum.org/article/20071...al-studio.html