I'm sorry Jan, it's not possible to run Infinispan on Java5, it relies on many advanced concurrency features which where not available before.
BTW I'm sure you know that Java5 reached end of life?
A possibility would be to run Infinispan in it's own VM using Java6, and connect to it via HotRod. At the moment as we thought that nobody was still interested in Java5 also HotRod is built with Java6, but it might be an easy port, if you're interested to contribute a patch. Of course, it won't be as fast as an in-VM Infinispan.