I can't answer you directly since I don't use Hibernate directly, but I can point you to something that might help. The Spring framework provides a support class for handling CLOBs and BLOBs. I'm using the BLOB support class to store 16kb byte arrays. Since it's open source, you can get the source and check out how they handled this.
subject: How to store a string(more than 4 KB) into CLOB data using hibernate