Hi Anne. Thanks for the feedback! We have had some success in crossing the developer\designer divide.
We’ve also had the occasional group-hack, not a hack-day, just a bunch of us getting together to discuss and build something that we have a combined interest in over a non-fixed period. One recent group-hack was to build a puzzle game, which we’re still working on. It’s a nice way to get designers and developers (and other departments, of course) working together and communicating. If you find it difficult to arrange a formal hack-day involving the whole company, then a smaller group-hack is ideal, and you can rotate members to get more exposure.