I know that as Vengeance, you actually receive a buff that gains stacks 1 through 5 as more soul fragments are spawned, so it's easy to track. I don't believe Havoc adheres to this behavior... on top of that, I don't even know if the amount of potential fragments out at one time is capped. It's not on kills, so I kind of doubt it.
I don't know if it's something you'd be able to track with WeakAuras. I'd have to enable advanced combat logging and see if there is any entry when one is created; if there's a reference to Demonic Appetite being used as a spell or the soul fragment being created. As far as I know, the only time you'll receive an entry in the combat log is when the fragment is absorbed.
Edit: I may have been wrong, looks like this guy made some progress:
http://www.mmo-champion.com/threads/...-Soul-Tracking
I would try using his import code and go from there. From what I read, he actually got it working and just had trouble tracking the souls from KILLS, but the ones from Demonic Appetite were, apparently, showing up fine. If so, that should work perfectly for you! If you can't get it working let me know and I'll import it myself and fix it up.
Here is a direct link to his most recent Weakaura, where apparently it's working fine for Demonic Appetite, just not for Shattered Souls (the ones from killing blows):
http://pastebin.com/1g6Hbtty