@Allanyed and Druxey: This is an interesting question. Crosspieces took terrific wear on their after surfaces and had to be changed out from time to time. In big seventeenth-century ships (maybe third-rates and above) the crosspieces were in two parts, a massive permanent piece which was bolted to the pins, and a much lighter piece which was probably hooked to the forward piece and thus could be conveniently changed as needed. In lesser ships the crosspieces were light enough to be manhandled, and thus were in one piece hooked to the pins for easy removal. My area of study is the seventeenth century, so I'm not sure whether this still applied in Nelson's time, but the joints of the after pieces are visible in Longridge's plate 23; are the forward crosspiece sections bolted or hooked to the pins? I can't tell from the photo, but I know the whiplashing of a violent explosion such as a bomb under the hull could shear off bolts, and do so in steel ships as well.
@Jaager: If numerous long iron bolts going through keel, floors, and kelson could survive for many years without rusting out, then I think those at the bitts could survive as well.