Since we had based a whole cruise on using our folding bikes (a repositioning cruise from Montreal to Tampa), we were pretty bummed after happening on this thread 10 days before departure. However, we are happy to report that there appears to be some flexility in the HAL policy on a ship-by-ship basis. While we received no explanation, after we contacted our HAL cruise consultant, she gave us the contact information for the HAL Ship Services Coordinator for the ms Rotterdam, and, in turn, after explaining our situation, the Coordinator told us told that the Rotterdam confirmed that "as long as you store it in your stateroom you are more than welcome to bring your bikes onboard with you."
Our facts: We have two lightweight, single speed folding bikes that each fit into a suitcase meeting airline size restrictions. As with any such suitcases, these fit under our bed (along with our other two bags). We kept our bikes folded in the stateroom and carried them off ship folded (we did ask once about bringing them back unfolded, but were told no, as bikes aren't allowed). Obviously, we could not bring them ashore on ports where we tendered, but otherwise we had a great time using them in Quebec City, Halifax, Sydney, Boston, New York, and Key West. We rented bikes when we were tendered in Bar Harbor (which is lucky, since they would have been useless to us with the hills in Acadia National Park).