You might try "boot -2 netbsd" to disable ACPI completely, in which case NetBSD would not be able to request ACPI S5 state to shut down and power off (would need to use older BIOS interfaces).
If your system is an older one (HP model numbers mean nothing to me) then it shoukd work OK without ACPI. You could also confirm whether other OS's are able to power off that particular system (the test needs to be of that exact system, as minor variations like what BIOS rev is installed, what other hardware, and the BIOS config all could alter the results. Installing other systems should not be needed, "live" cd/dvd boots (or USB stick) should be sufficient to test this. But testing NetBSD with ACPI disabled first is the quick test to perform. kre
