malcomhartill,
You should check the docs, specifically..
http://docs.splunk.com/Documentation/Splunk/latest/Data/WhatSplunkcanmonitor#What_kind_of_data.3F
There are many different ways Splunk can collect data, you just need to find the right one for your setup (whether it be a script querying an API, or log files output by another tool/system).
There is a Splunk "App" for AS400-iSeries...
http://splunk-base.splunk.com/apps/24097/splunk-for-as400-iseries
(But once you have collected the data, you could always create your own App)
Regards,
MHibbin