There is no method that I have seen similar to using named pipes for export derived to the RMAN backups. RMAN does a form of compression when you do BACKUP DATABASE (not an incremental backup) by not backing up any blocks that are empty. But your backup destination needs to have enough space. Many who do not have enough disk space perform one of three options: 1) backup directly to tape, 2) backup to a network attached disk device or 3) procure more disk.
Let's say that you were able to perform some sort of compression and saved 20% of the required space to back up your database. Then what happens when your database grows another 30%? You'll be out of space for your backups again. I'd solve this issue without compression because the space shortage can come up again in the future.
This was first published in April 2006