Hello,
being the one who wrote the Prelude support in OSSEC and now working at Splunk, I believe I can help you 🙂
In order to make OSSEC send data to Prelude, I had to convert the alert output into IDMEF, using the Prelude IDS library. You can see this in this file: https://github.com/ossec/ossec-hids/blob/master/src/analysisd/prelude.c. You can see more details on what I did there: https://groups.google.com/forum/#!topic/ossec-dev/sYXnT6-LgI4
The approach Splunk has is not to alter original software to work with their data and be agnostic to index them. In order to get the fields (source ip, attack name etc..) we apply a datamodel at search time, allowing us to do many things, such as map-reducing and get the results much faster.
Splunk was designed to scale (by that I really mean it!), provide a framework and ready-to-use apps out of the box.
I recommend you take the OSSEC Splunk app (http://apps.splunk.com/app/300/ ) so you can benefit from already cooked dashboards.
