The deceptive thing about hammers is that you want high pressure, like in a bec-de-corbin. Lead's very dense, which is a plus, but won't hold any kind of even slight point, which is a minus. Even a rectangular head would probably deform in use against armoured targets. It would be slaughter against unarmoured people, but then so would many metal warhammers.
Also, it's worth noting explicitly Arx has implied: the act of deformation itself will sap the power of the impact even further, even above and beyond the distribution of force caused by the increase in surface area created as a consequence of deformation. When you hit something with armor, you want something hard enough to withstand the impact not only so that you can use it more than once, but also so that you actually deliver that kinetic energy into the armor instead of your own weapon.
I agree with a lead core being your best option for weaponry. If you use wood as your casing, a loaded shillelagh is almost traditional in that role, but the steel pipe already suggested is probably easier to come by nowadays. Pure lead can also be used for sling bullets as well as firearms, but I would check your local statutes for that as many jurisdictions ban lead bullets, and wording on that sort of legislation can be a bit tricky.
Also, the only manner in which people will be dying of aluminum toxicity or lead poisoning in the near term would be an excess of the metal rammed into their skull, through their heart, or whatever other vaguely-sarcastic turn of phrase you prefer. These tend to work on longer terms than a single battle. ^_^