I promise you, there is strong demand for this software. Audio drama writers desperately want scriptwriting software.
The format has to look like this: http://downloads.bbc.co.uk/writersroom/scripts/bbcradioscene.pdf
It HAS to have line numbering. The sound effect directions often include stuff like, "rain continues through line 45" and so on.
Shortcuts, like you find in Final Draft and Celtx, are needed. It's easier if we can hit tab twice, type "b" and get "Brienne Of Tarth."
Essentially, Celtx needs to get off its ass and bring back its radio drama script format (which I think they 86ed) at a reasonable price point, or a disgruntled Celtx employee needs to rip off the software and make an open source version on the DL. Final Draft is the most commonly used software in the US for scriptwriting, and they've never had a radio play format. They need to get with the program.