I'm photographing professionally for a few years now, and I'm starting to feel some arm pain, I think it's related to long shooting sessions. I'm seeing a doctor in order to diagnose and treat it, but I'm also wondering if is there some accessory to attach the camera to my right arm, so I can easily shoot with my right hand but don't have to hold the weight of the camera with my hand for hours.
A lot of amateurs use a wrist strap that helps hold the camera to the right hand. In 30+ years of moving around in photographic circles, I've never seen a single professional photographer (defined as someone whose primary income is derived from taking still pictures) using one. Ever. They are available in a wide range of materials, designs, and prices.
One of the first things most shooters learn in photography school is to support the camera with the left hand when shooting handheld. The right hand is for operating the camera's controls. The left hand is for holding/supporting the camera as well as using the fingertips to operate zoom and/or focus rings if necessary. The overwhelming majority of still cameras are designed to accommodate this basic concept.
If you are holding the camera correctly you should be able to completely remove your right hand with no loss of support of the camera. If you're supporting the weight of the camera with your right hand from the side of the camera, rather than with your left hand underneath the camera, then that is the source of your pain problem.
Video cameras, on the other hand, almost universally have a wrist strap that helps the videographer keep their right hand held to the controls. But pro level video cameras rest on the right shoulder (or a more involved stabilizer system such as SteadiCam where the cmaera rig is strpped to the body) for supporting the weight of the camera, so it is a totally different technique.
Just as there is a "standard" way to swing a golf club (or many other physical motions) that will normally result in best results, there is a best practice that will usually result in minimizing camera movement and maximizing the ability to control the camera quickly and efficiently.
Of course there are special cases for someone who has a physical issue that makes it difficult if not impossible to use normal technique. In art school they teach you to hold a brush a specific way for making a certain type of stroke. But there are successful artists who paint with their feet or even by holding the brush in their mouth because their arms/hands are not functional.
Without more detail it's hard to be confident that the problem is to do with weight. It might be posture. Consider that most cases of carpal tunnel syndrome are due to bad posture at a computer, where it's the angle of the wrist and the repetitive movement of the fingers rather than the weight supported which does the damage. And I can tell you from experience that even though the actual injury is in the wrist, the pain is felt all along the arm.
However, with respect to relieving the weight and possibly also improving posture, I think you're too quick to dismiss monopods as a possible solution. I was at a wedding last weekend where one of the photographers was using a monopod and live view and seemed to have no problem moving around the room.