I hacked Splunk/SSO to let users auto-login as the "splunk" user using the following Apache config (you'll need to ensure that this user exists on your Splunk instance).
Listen 8989
<VirtualHost *:8989>
<Proxy http://127.0.0.1:8989/*>;
Allow from all
</Proxy>
<LocationMatch "/">
ProxyPass http://127.0.0.1:8000/
ProxyPassReverse http://127.0.0.1:8000/
Header add REMOTE_USER "splunk"
RequestHeader set REMOTE_USER "splunk"
</LocationMatch>
</VirtualHost>
Then in Settings > Server Settings > General Settings set "SSO Trusted IP" to 127.0.0.1
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