Thanks andowson. After some debugging, it looks like PostAction.insertSave is redirecting the path. So I didn't change any of its code. What I ended up doing was in the PostREST, I retrieve the post link from JForumExecutionContext.getRedirectTo() and then set it to null with JForumExecutionContext.setRedirect(null). That seem to do the trick.
Here's a sample snipplet of my modified PostREST.java
public void insert()
{
...
final Post post = new Post();
post.setForumId(Integer.valueOf(forumId));
post.setSubject(subject);
post.setText(message);
this.insertMessage(user, post);
// new start
String postLink = JForumExecutionContext.getRedirectTo();
JForumExecutionContext.setRedirect(null);
this.setTemplateName(TemplateKeys.API_POST_INSERT);
this.context.put("postLink", postLink);
// new end
SessionFacade.makeUnlogged();
SessionFacade.remove(sessionId);
...
}
Thanks!