Install 32bit Java 7 OpenJDK
It seems I found a source of precompiled OpenJDK:
http://code.google.com/p/openjdk-osx-build/
The OpenJDK 7 builds there are universal, and do indeed run in 32bit mode.
The latest version is 7u6, which is not quite as recent as the 7u7 available from Oracle, but should be recent enough for my requirements.

Alas, my original problem persists, so I might have to compile my JVM myself after all. I found there is a jdk7u-osx repository in the OpenJDK mercurial repositories, and its README-builds file provides reasonable instructions on how to build stuff. The part about source drops appears particularly important. I still have some troubles, but those seem to be bugs in the build process or similar, so I'll take them up with the macosx-port-dev mailing list.