Well, good news and bad here…
The good news is that, with the DSDT fix (although in all honesty I’ve not tried without) the 10.5.6 upgrade does the right thing and reboots correctly. Even better, a modified SystemConfiguration is no longer needed in /System/Library. May be due to this, the screen is also no longer stuck at full brightness… although due to the following issue, I don’t know whether it can be modified.
If running Mac OS on a non-Apple laptop, then there’s a sting in the tail of the forthcoming 10.5.6 update: it appears that the new kernel only wants to run with ACPI HPETs, and will fail on boot if legacy i8254/RTC timers have interrupts assigned to them.
To fix this, a replacement ACPI Differentiated System Description Table is required. efi_boot, version 6.1 or above, has the ability to replace the DSDT supplied to the kernel from the system firmware with a customised one: Simply place the alternative DSDT.aml in the root of the EFI volume – generally /Volumes/EFI/.