I recall asking for this in an RFE sense a year ago as it would be incredibly useful. Sun just put the capability to track the load on disks at the partition level in Solaris 2.6 - this sort of stat on a filer would be just as useful IMHO
Per quota tree seemed most natural to me as that is already a distinctive structure in filer housekeeping meta-data. Directory level gathering might be too much load to put on a filer which is meant to perform under high demand...
...how about an admin command that flips a bit on a directory and makes it a watchpoint in the filesystem - accesses against anything beneath it count on that directory's stats. Even if the total number of such points were limited to put a boundary on overhead in the filer, it would still be a useful analytical tool.