Thanks, you solved two mysteries: whether finalisers were originally intended to raise, and why GC alarms ended up in Pervasives (there are no examples in the docs, no use in the compiler, and the git logs give no explanation and refer to no issue on a bug tracker).
Zenon indeed made a blip on my grep radar, but I erroneously dismissed it as another program that only use alarms to print or log something. It shows me how hard it is to read a function and try to determine whether it raises or not.