Searching GMANE (a mailing list archive service) seems helpful, it yields (among others) `gmane.linux.acpi.devel`, the _Linux ACPI Development Discussion List_. While I'm not sure if it's where you'll find the developers of `acpid`, it is mentioned there, so it might be worth a try.
**Edit** Looking at Debian's packages for the homepage of some project is often helpful, too.