Actually, not being in the supported list means that : It's just not tested
Only the file-systems we actually test on are listed as 'supported' in the docs as those are the one we KNOW to work with Splunk. All other file-systems will likely work, but it's on a best-efforts basis only. Any fs-related issues that pop up will be pre-cluded from an fix/workaround SLA commitments. In order to run on an unsupported file system, you have to bypass the 'locktest' check that runs on startup – a symlink is the easiest way to do this.
As always, our support agreement is with the Customer, so we will do as much as possible to help them get up and running, but if they choose to use a file system that is untested & unsupported, we can only do so much.