There's no reason I can think of to suspect that the Ring wouldn't work for an Ent. The Ring was an instrument of "command and domination" forged by Sauron to dominate the wills of his servants, the holders of the other great Rings, and anyone else who crossed his path. (He also used the Ring to dominate the Numenoreans.) There's no reason an Ent wouldn't have also been able to use this power.
Given that it fell into the hands into a less-than-good Ent (unlike Treebeard, who probably would have refused it) he would have had to take some time to train himself to the use of the Ring. For while the Ring immediately offers:
It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power.
It takes some time to actually be able to use the Ring for your own purposes, effectively.
'Only thrice have you set the Ring upon your finger since you knew what you possessed. Do no try! It would destroy you. Did not Gandalf tell you the rings give power according to the measure of their possessor? Before you could use that power you would need to become far stronger, and train your will to the domination of others.'
(Lord of the Rings)
He [Frodo] needed time, much time, before he could control the Ring or (which in such a case is the same) before it could control him; before his will and arrogance could grow to a stature in which he could dominate other major hostile wills.
Such an Ent would have quickly realized that he must first eliminate the threat of Sauron reclaiming the Ring. But as Sauron was more powerful than any Ent, and the Ring was rightfully his, a direct confrontation would have been futile. So most likely, he would have raised up an army of trees and Ents:
In any case Elrond or Galadriel would have proceeded in the policy now adopted by Sauron: they would have built up an empire with great and absolutely subservient generals and armies and engines of war, until they could challenge Sauron and destroy him by force.
Confrontation of Sauron alone, unaided, self to self was not contemplated. One can imagine the scene in which Gandalf, say, was placed in such a position. It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors. If Gandalf proved the victor, the result would have been for Sauron the same as the destruction of the Ring; for him it would have been destroyed, taken from him for ever. But the Ring and all its works would have endured. It would have been the master in the end.
Probably, an Ent-King's problems would be exacerbated when all the peoples of Middle-Eath realized it was their Enemy, so the war would have been fought on two fronts and therefore perhaps to less success. Or perhaps the free peoples of Middle-Earth would have let Sauron and the Ents duke it out first. Who knows?
Ultimately, the Ent would have become wholly evil, as the Ring became master. And it would have indeed likely ushered in the age of the trees, since that's what Ents love. It would not have been a good time to walk on two legs.
As for the subsidiary powers of the Ring (apparently the side-effects of being distilled Sauron), it's less clear. Would the Ring have made an Ent invisible? Hard to say. Would it grant them understanding of speech? Likely, since that seems to have been directly related to the primary power. Long life? The Ents were already very long lived, and I see no reason it wouldn't preserve them as it would most anything else.