You’ve answered your own question. Version 2 plug-ins support text (strings) boxes.
The plug-in interface of Nyquist plug-ins is not actually part of Nyquist (or XLisp). The interface is part of Audacity. Nyquist is just the code that you type and the number crunching and sound crunching that goes on invisibly behind the scenes.
The interface options for Audacity/Nyquist plug-ins are very limited and are all described here: http://www.audacity-forum.de/download/edgar/nyquist/nyquist-doc/devel/audacity-nyquist-en.htm
It would be nice to have a bit more flexibility with the interface, but until someone invents a more flexible interface we are stuck with what we’ve got. On the other hand it can lead to some very creative thinking to find solutions to problems.