Had a think about it and I think I figured it out--my problem is a quirk with using local feeds.
The trouble is that there is an issue with ipkg, where if you ipkg update and you don't have a feed that lists the original system packages (ie. all of the ones that come with the ROM), ipkg thinks that those packages are not installed at all (and complains about missing the dependencies for absolutely everything).
Thus, I am using the Packages files straight from the feeds (so that ipkg has feeds to look at which contain all of the system packages), but have only actually downloaded the .ipks that I need, so there are a lot of packages listed in the Packages files i'm using which aren't actually contained in my local feed. If I forgot to download one of the dependencies, ipkg tries to get it from my local feed, thinking it is there because it is listed in the Packages file, and then quits immediately when it fails to copy the package.
Ipkg does not (probably by design) check whether the files it needs are actually there when doing dependency checks, and if it tries to copy a package from your feed that isn't actually there (but it is listed in the Packages file), it just quits, without even attempting to install further packages.
If only there were a better way to do this offline...
I need a network card for my Z...
EDIT: I checked the logs. Suspicion confirmed. I was actually not missing any depends after having retrieved the one it asked for last time (which I later got and installed manually)--all the depends listed after that one didn't get installed because of ipkg quitting. File manager works. Huzzah. File this one under "you really, really, really, really SHOULD buy that wifi card.