A quick test suggests that CALL fgl_report_setTitle() can be used to override the value in the .4rp and appears in the Chrome of the Genero Report Viewer, and in the Document Properties of a PDF Report.
Reading the documentation, it could possibly be worded better, but I don't think it says that these functions only apply to ASCII reports (where there is no .4rp and hence no other way to set these values), but instead it is trying to say that the best place to set these values is in the .4rp.
An issue I do see with using the fgl_report_setTitle(%"title") or fgl_report_setTitle(LSTR(title_variable)) is that the 4gl is responsible for the title and so would need to have it somewhere where it can find it, which could be an issue if you have wrapped up most of the fgl_report calls in a library. I could see how being able to set the metadata in the .4rp as an RTL expression such as "report.title.invoice".translate() would be advantageous.
Reuben