As coaster has said, it's unusual for snow to persist on Macdui. That said, the last time it persisted was 2002 (off the top of my head) I think, at Garbh Uisge Beag. I believe the location was the one in this photograph: [forum.winterhighland.info
] but I'll verify that for certain.
If the weather cools down in the second half of September and we get some decent early snow then this patch could well survive, but it's long shot I fancy.