You could get someone mining until they have high stats, then danger-room him, then give him the best of whatever equipment you have. Would it be enough?
A good idea that, with some tweaking, could evolve into something workable. The real trick is that all this cast-off stuff probably isn't going to cut the muster, and most of that is on the surface anyway. The thing that really needs to be secured is an anvil (if not secured already; didn't see reference to such). Hopefully there's one laying on the surface somewhere and not near any of the zombie clusters, and if there isn't one currently, there probably will be one eventually. It would be worth the lives of a handfull of dwarves to get an anvil down into the safety of the lower-fort.
If/once the anvil is secured, then achieving the magma sea becomes priority. 90% of the time, once you hit the sea, you also find candy. With any amount of luck (in short supply, I know) that candy will also be in the stone layer. 30 pieces of candy is enough for one dwarf. Even without a legendary candy cooker, it is probably the best bet of reclaiming, and more importantly holding, the surface.
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You know, a crazy way to possibly get the anvil just occurred to me and it would have far fewer risks than running across the surface to drag back the anvil. Now that the area area below the aquifer is controlled, a cave-in could easily be staged! The only dodgey bit would be channeling the surface. That would have to be done fast and possibly while there was a diversion (migrants).
Dig out an area under a caravan remnant that has an anvil and then collapse it into a fully enclosed chamber. The aquifer will quickly form a water barrier preventing any zombies from following it down, not that it would be a problem. Now, wait for winter and it'll freeze solid (or at least solid enough to easily drain out anything that isn't frozen). From there it's just simple ice-reclamation digging.