For the ACPI case, I still think that an AML call from the AHCI driver
is the most logical solution. You mentioned that you believe that calling
into the AML interpreter up to 100 times per second is a noticeable
overhead, but I doubt that and would like to see actual number backing
that up. Note that most of the time, the status of the LEDs won't even
change, so the driver does not have to call into the AML while I/O
is in progress, or while it is stopped, only for the transition or in
case of locate and fault events that should be extremely rare.

It can also be delayed - you don't need to adjust the LED at any great
rate - it's for a human not a machine to read.

Alan