The item can maintain concentration.
But, IIRC, to control the spell, one needs to be in range of the item. I believe this was quoted recently in another thread... *
It could thus be argued that this doesn't give the grogs the power to fly, nor can they direct their flight, and that separated from the item, a grog would just follow the last command given to the item or hover in place.
Then, to give them the power of flight, you'd need a MuVi effect like Passing the Reigns of Corpus.