Shame on me for not reading instructions.
After reading nukedathlonman's post (#167) a little closer, I tried a different version of libzip.
The one available here,
http://packages.debian.org/sid/i386/libzip1/filelist, I created a link from /usr/lib/libzip.so.1.0.0 (installed by the package;
not the one in /usr/lib32) to /usr/local/Kobo/libzip.so.1. Now it works. Hurray! :-)
(For the sake of completeness, I actually made a link named /usr/local/Kobo/libzip.so.1.0. That worked. I just renamed it to libzip.so.1, but since there was nothing to sync anymore, it might not work anymore. But at least I know that a link named /usr/local/Kobo/libzip.so.1.0 works.)