when I transition from CC to OC (i.e. when entering a restaurant that sells alcoholic beverages for on-premises consumption), I tuck my shirt in behind my IWB-holstered Taurus 905. There's not much else I can do to unconceal my sidearm any more than that.
Unless I were to put the holster between the belt and my pants, but that's just silly.
Wouldn't it be openly carrying? That's all you can typically see of a holstered sidearm anyways, whether the holster is OWB or IWB: the grip, and the end of the frame. If you can tell it's a sidearm, and the manner in which it is carried doesn't obscure the fact that it's a working gun (i.e. the mini-revolver belt-buckle -- http://www.naaminis.com/custcry.html ), then it's being openly carried, IMO.