Yes; my take is that they aren't very common, but I have no direct personal experience, so take this with a grain of salt... Since the order is "hidden" it's not sent to the L2 book, so unless your broker (i.e. Datek, IB) routes heavily to ISLD, I wouldn't think that the hidden orders are very marketable. I mean, what's the point of making an offer if no one knows it's there? If a tree falls in the woods and no one hears it, did it really fall??
It's always been strange to me that ISLD, whose originators have been champions of market transparency and liquidity, is one of a very small # of marketplaces that process hidden orders...