There was a mistake in my bash script: I added *.java to the jar instead of *.class. Stupid me. Another, minor mistake was that I named my key aliases as xy$PROJECTNAME. I should've used xy$USER, because I obviously need only one developer key triplet, to be used for all my projects. It still wouldn't work, though, and I learnt that the manifest file in the original post was wrong. Here's one that actually works:
Implementation-Title: Hello World!
The lines Main-Class and those concerning the SDK seem to be important in particular, but are not included in the manifest shown on the first post in this thread. Somebody please update that post!