Your problem is in no way JBoss specific.
You have a number of options open to you:
1. Write to a shared file system(e.g., NFS, Samba, etc)
2. Write to a JDBC database
3. Use JBossCache - write the contents as Objects (or maybe byte's if they have no specific Java representations). JBossCache can then be configured to replicate contents across server instances. And if you wish, JBossCache can be configured with a cache loader (see manual) so that the cache can be backed up to a file system, database, etc.
Once written to disk, are these files accessed by other systems?
JBossCache (perhaps backed by a JDBC cache loader) would be ideal for this then ...