Just a followup:
I found an "interim solution" by using a String type instead of my LongString type and mapping it as jdbc-type CLOB and sql-type TEXT (a mySQL type).
The Jboss code apparently treats String as a "primitive" class and uses toString() instead of Serialization to store the field value.
This way I managed to store the field without the Serialization signatures.
I could have optionally patched the source to treat my LongString differently but that would be a "dirty" solution.
Can anyone tell me if there are plans to support *custom* serialization of custom data types?