Bugger, broken drill bit... the conclusion...
 

As it turns out, there are several different chemicals all generically
sold as "Alum" and not all will do the dissolve out the drill bit thing.
At any rate, the one I tried didn't. After a bit more fiddling I decided
it just wasn't worth the effort to try to remove it, and I'm getting KNS
push button pins and forgetting about the drill bit remnant.