Well its really MyBatis which parses its own configuration file. So I suggest to look at the MyBatis project and see what kind of property placeholder functionality it supports (if any).
I think this is can be a common use-case for others, so fell free to keep us posted. Maybe if MyBatis has some hooks we can integrate that with Camel's property placeholder. Or let camel-mybatis pre-parse the configuration file, and replace placeholders first. Though the latter is a bit harder as we can only do a textual search/replace. Where as MyBatis better understand it's own configuration format than we do.