YOU own, the photos, not Mr. Rock or the radio station regardless of whether they granted you "media" access. Access to the backstage areas does NOT constitute "payment" so he can't even say he "bought" the photos.
Lacking any kind of written contract, or any mention of"exclusivity" in any verbal contract you MAY have with him, the photos are yours and yours alone. Selling them is another question which may not be a problem for the reason you think.
1. In a funny way, the fact that you have not been paid actually works in your favor in this case. Had you been paid to shoot the event, the station or the venue could claim ownership of the photos as the product of the work they paid for. Hard for them to claim ownership of something they didn't pay for. And again, access is not generally considered "payment."
2. I'd tell Mr. Rock that you want to sign a written contract this year to provide the station with a limited usage license to the photos you shoot in exchange for complimentary access to the event with full access to the backstage areas. Specify that the photos can be used by the station only for promotional or advertising purposes, but that sales of the photos to 3rd parties must go through you.
I'd still do the event for free, because access to the backstage is a valuable thing and allows you to get photos that the general audience cannot shoot. Your payment will come thru your subsequent sales.
3. Finally, the MAIN pitfall in all of this may actually rest more with the performers than with the station. Depending on what their contract with the station says, you may or may not need some kind of licensing agreement with them to distribute/sell images of their performances. This is different from a "model release" in that many performer's images/persona's and acts are actually copyrighted or trademarked in and of themselves. Thus selling photos of a performance could violate THEIR copyright (not a nice thing for one artist to do to another). Also, unlike a model release, which is generally only required for "commercial use" (such as in an advertisement or similar situation) but NOT for "editorial use (which can cover a whole lotta ground), copyright licensing applies to ALL uses of the person or entity's image.
For instance, people making t-shirts picturing Michael Jackson and then selling them on the corner are violating the copyrights AND trademarks protecting MJ's image.
In the end, it would be in your best interest to
talk to a lawyer as Wheatfield suggested. Just make sure it is a lawyer who is familiar with intellectual property/trademark/copyright law. Don't bother with the ambulance chasers or the "I can get you out of a DUI" guys.
Best of luck and if Mr. Rock balks at your new conditions tell him fine and skip the event. They will feel it when they only have crappy P&S photos to show for their trouble and they'll probably call you back next year.
Mike