WD40 might work, but it also might well not. The aluminium/steel reaction is pretty powerful, and I've had seatposts I couldn't extract even after making a large wooden clamp and hitting that with a big mallet. The C&V forum guys know about this better than anyone, but with seatposts, the final solution is to melt the post with lye, which is dangerous both to you and the frame, and very unlikely to work in a hinge bolt. The moral of the story is that wherever an aluminium component touches steel, it should be greased, but this is a message to Dahon, not you!