Yes, this does work on a per document basis, but what I request is a builtin feature that simply applies out of the box and is backwards compatible.
it's not clear what you want to achieve.... What does "busy" mean in this context ?
It means that the output file is named jobname.pdf despite it is known to be incomplete and unusable during all the typesetting process.
The output file should be named jobname.pdf only once it is really a complete pdf file.
In the meanwhile its extension is pdf(busy), which means that it is a pdf file under construct.