If there's a JDBC driver for it that can be installed into a Tomcat, then yes, you can use any database you want. Of course, you won't be supported unless it's one of the four Atlassian do support. I don't believe that there are any plans to support anything other than the current four databases (barring maybe adding Maria as MySQL is going that way)
But, bear in mind that JIRA needs it to look as though it's a single database, which it owns and has exclusive access to. You can cluster the back-end database, but it must look like a single database server to JIRA (You can't cluster JIRA, unless you go to the Data Centre installation, and that pretty much requires a supported database server)
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