The only reference (and anecdotal) that I could get was from Zam, but I believe the drop rate for "Sarnak Hatchling Brains" may be off.
http://everquest.allakhazam.com/db/quest.html?quest=97
The brains are supposed to be more common, as per the comments. Many report getting 50 - 75% drop rate. I killed 30-40 hatchlings tonight and only received 1 brain drop plus one more someone had left on a corpse.
I already have received the pieces for skull cap #2 5 times over, and I believe those two tapestries are supposed to be the rares off the sarnak hatchlings.
-Crover/Yolo