We have the necessary experience, persistence can be based on RDBMS (MySql, for example) and mapped to SOAP with our own XML schema. You might consider native xml database so as to avoid mapping to relational, since most of the transaction is in xml.
Also, will provide UML design (see profile) so that the entire applet/application can be easily ported, or be able to support multiple interfaces and/or architecture.
Please feel free to contact via PMB.