VST GUIs are generally a problem for screen readers. The only chance to get access to the controls are VST hosts which allow a direct access without using the plugin's own GUI. Audacity and reaper can do this for instance, but not RadioBoss and RadioCaster.
As an alternative to stereo tool you can try JB broadcast, downloadable from the DJSoft.net homepage. The GUI isn't accessible too, but there are some built-in presets available which are suitable for most use cases. RadioBoss however can't select these presets or can't select them in an accessible way, but with a combination of VSTHost and VB Cable you'll get a half-accessible solution.