Is it possible to select alternate characters of an OpenType font in scribus? If so, how?
At the time of writing, this is -- at least partially -- possible in the Scribus development version 1.5.4.
You will find it in the new Text Properties Palette, in the "Font Features" tab.
You can get the development version for Mac and Windows (and an appimage for Linux) from the Scribus website and many Linux distribution provide channels / repositories with nightly builds (for Ubuntu there is a PPA).