Your right. You just have to add "interface" between your Qt widgets and rqt plugin framework. For rqt plugin in C++, only the class that extends rqt_gui_cpp::Plugin is rqt specific. So create it, and use your custom Qt classes from there.
There may be only a handful of plugins written in C++. One good example is rqt_image_view. See its code as a reference (note that this plugin, however, is written all-in-one style of Plugin class and Qt widget).