Or have I been wasting my time because there's a way to get the ACPI-functionality built into the kernel to work?
As far as I know, ACPI is mandatory for amd64 and it is compiled directly into kernel. "sysctl -a | grep -i acpi" should show some related kernel variables. May be some power management module have to be loaded, see "ls /lib/modules/*/*pm.ko". Hope it helps. Petr