I got my PGWP by flagpoling (indeed not exactly flagpoling, but I did a day trip in the US and applied for the PGWP on my way back at the PoE). It worked. Back then, my Study Permit was to expire in 2 more days. Everything went very smoothly if you got all the required documents in order (and no need to fill in the form).
Importantly, you need your passport, still-valid Study Permit, most updated official transcripts which show the grades of last semester and that you have completed all requirements of the programme, a letter from the registrar of your institution that states clearly that you have completed all requirements of the programme (and with a note saying that you have been registered full time from xx to xx --bcoz I did a research degree that involved dissertation, the transcripts show no grade or course taken during the dissertation writing semesters... and I was also asked about it at the PoE).--- I think those are all that are needed (hope I did not miss any item).
But I have a small concern that you have been refused (though erroneously) for this same application by the CIC. I believe you will be asked about it. So I would advise you to best prepare to explain why that refusal was a mistake. I am not sure if that refusal would matter for flagpoling. But yes, this will be quick. It takes less than 20 minutes. Good luck!