Look into seam-gen/view directory.
You can modify them anyway you want. Seam-gen is using FreeMarker for templating.
It is driven off code in the hibernate tools package and uses freemarker. Look in the
seam-genfolder of jboss-seam. I found the templates to be pretty straight forward, the harder part for me was understanding the hibernate api that drives it. If it's only the templates you need to change, I think you'll find it's pretty easy. In my case, I wanted to put a custom annotation on an entity and use that to drive some templating. I couldn't figure out how to check for my annotation in the templates.
Personally I think one can be easily tricked into following this path. You invariably find so much other stuff you need to change after the code is generated. But if you have lots of entities and the template changes are easy, it could pay off.