I want to install Splunk to run under Redhat Linux 5 running on an IFL processor under z/VM on an IBM z9. I see this same question has been asked for SUSE and s390. I'm asking it again for Redhat support on s390. My sales team (today) has opened a feature request for support on s390.
There is a fully supported s390x Universal Forwarder on the Splunk forwarder download page under Linux in tgz and rpm format:
http://www.splunk.com/en_us/download/universal-forwarder.html
We're also interested in a s390 zLinux/zVM port of splunk. We're currently running >370 virtual Linux machines on our System Z machine. This shouldn't be too hard to do...
Make sure you communicate this requirement to Splunk, either through the Enhancement Request process or your sales representative. (Or both!)
Today there is no support for any operating system under the S/390 architecture. Filing an enhancement request for this is the right next step - which it sounds like you have done.
All of the various Linux distros on S/390 architecture will be a substantially easier port than Splunk on either (native) z/VM or z/OS.