I have never heard of this being used in vivs, but I can tell you millet is sold for birds to eat. We have some for our parakeet, who seems to love throwing it in his water dish and when it gets wet, yes it is soggy and once wet I have never seen it dry out, however we do clean his cage twice a week so maybe after a week or so it may dry out if we would leave it in there.
So as suggested already, I would think it would retain moisture and either bloom or rot like grass seed etc. does when it sets in moist environment. :shrugs: