Well, Mitsukuni Haninozuka is known to carry around a stuffed rabbit, which is essentially a plushie. It is a pretty defining feature of his and should be considered a cosplay prop or accessory.
I do agree, though, that most plushies are considered non-cosplay items. In the end, it's at a moderator's discretion how they want to enforce the no non-cosplay item rule.
New Cosplays for 2015
Steampunk World's Fair?
Steampunk World's Fair