Ah grasshopper, never forget who signs the front of the check and who signs the back. The reason HR is resistant to TPR's is because of the cut throat, pushy, hot shots in our industry. Play by the rules , build your relationships with HR, become part of their team and understand that part of their job is to find those candidates themselves when at all possible.
In a new situation I make a deal with HR to let me submit candidates , if they can find the same ones I let them have them with the understanding that they will include my good ones they didn't find along with theirs when candidates are submitted to the hiring manager.
It won't take long before they see you as an addition to their team not a competitor. It also won't take long before they come back to you asking for help on a difficult search or when the internal time constraints are a press for them. It won't take long before they trust you not to undermine them and will ask you to speak directly with their hiring managers when they need your help. HR can be your best friend for long term relationships and your worst enemy if you go around them. I try to find out who the toughest hiring manger is within an organization , the one who gives HR hell. If i can help them with that manager's reqs they are more than happy to turn me loose to work direct as long as i keep them in the loop. It's their job to keep up with all candidates for lots of reasons. Because of the snakes in our industry it sometimes takes longer to build those relationships but it's worth it in the long run to be one of the "good guys" who get those listings when nobody else does because HR can trust you to help make them look good.
Consider this, if HR went around you to your boss asking for candidates and wouldn't work with you it would probably make you furious.
The stories of TPR's calling hiring managers to try and get a job order, then when they are referred to HR, calling HR telling HR that they have been referred by the hiring manager as if it were a personal relationship with the hiring manager are legendary. It may work sometimes but normally doesn't build long term relationships. If HR is going on vacation in the middle of a search ask who you can work with while they are gone or get permission to work with the hiring manger for that period of time.
I made a big placement this morning. When the offer email came from the VP of HR it was addressed to me and two internal HR people. It said: To the Team, please make this happen today. I was delighted to be included as part of the internal "team". I am working four 100K + positions with this HR group as we speak so give some thought to becoming part of the team.