I would suggest an API-function:
Code:
string filenames, integer timestamp = reaper.GetLastRenderedFiles()
which returns the rendered files from the last(!) rendering of files.
filenames - all rendered filenames including path, separated by a newline,
like: path/filename1.wav\npath2/filename2\npath3\filename3
timestamp - a timestamp, that signals, when the files were created,
so I can render files numerous times and still see, that
they were rendered again. To make it easy, reaper.time_precise()
would be the best value to put into there.
With that, it would be codeable for us.