If you are told by the Hiring Manager to "Work through HR," the following reply will usually disarm them:
"Oh, I'm sorry. I thought you were the Hiring Manager for the _____ area (or department)."
Ego is a powerful motivator. We all have it. And we react positively to those who stroke it.
Even the smallest ego cannot resist proving to you that he or she is in fact the person who makes the decisions regarding hiring for the department. Even if you are forced to follow this route by some arcane company policy (they usually don't really exist), make sure you reference the Hiring Manager in your cover letter and "cc:" (carbon copy) the person when you send your resume. This will enhance your image in the eyes of HR (since you already have the name of the Hiring Manager) and will leave the door open to this person in case you get lost in the HR shuffle.