I found the paper. I haven't read through it so I can't say if it is actually any good, but it is good enough that the FDA changed their recommendations because of it (that might not mean much, of course).
Thanks for tracking it down. Doesn't seem to be any problem with accepting their results that it's almost as good as a nasopharyngeal swab, so you won't see me complaining about it again.
https://www.medrxiv.org/content/10.1101/2020.04.01.20050005v...