Hi Martin,
How you redefine GetEntity and GetEntitySet same way you have to redefine Create and Update. Once redefine you have to write ABAP code for the logic which you need.
Go to http://scn.sap.com/docs/DOC-16326 ( Page 22 )
The above document will help you to write ABAP code inside the create same way you have to add logic inside update also.
Please let me know if you are facing any issue.
Thanks,
Arun Chembra