The file Documentation/leds/leds-class.txt provides no hint whatsoever about the use of the leds neither from userspace nor from kernel drivers that would like to implement triggers. Presumably the reason to implement this framework to prevent everybody implementing a led driver hiding the feature in some obscure corner of the kernel. Now some leds features are unified in this framework but what use is that without any docs?
Well, maybe all the features that can be used are documented and are just insufficeint. At least the features usable with the current userspace interface. There is just no way to use the LED framework in the situation when you have 4 disks and 1 led.
I looked at the led framework closely and it's not lack of documentation. The led framework just does not have code that would allow using a led as disk led when you have more than one disk controller.