Maybe I’m not quite following what you are asking but isn’t this feature there already? Click on the project or sub-project and then click ‘Download all time entry data’ and you get the time entries for that particular project.
That does work, but its not filterable (i.e. by a given calendar month). Also, the output order is all over the show when you do that.
Having said that, it is what I’m using at the moment. However, I have to convert the dates into the correct format (Klok doesn’t use ISO standard time/date formats in its output), then sort the output, then scrape out only the month I want, then do some other tweaking, THEN it goes into an invoice. Fortunately, this is all done programatically… but its shouldn’t be required.