What you can do is weld two flat bars on to the rollcage which forms a 45 degree angle (or there abouts) when the two bars meet coming out from the rollcage. The top bar is the one which the camera will be bolted on to. Also as a backup wrap the camera in a fire extinguisher metal strap to the top metal bar.
there is a bracket for sale in the demon tweeks catalogue.. it will pretty much fit any cage no matter what angle the cage is at.. i used it for the last two years and it worked fine had it in three different cars and there was no problem in setting it up.. as for your own camera, you can use your own provided it is in the car for scrutiny (also have to say your carrying a camera on d scrutiny sheet).. the only rally in ireland that you cant carry ur own camera is the ulster(international).. thats because greenlight have all rights to every bit of video filmed.. think you have to pay them money if you want to run an in-car camera.. but you definitely have to ask coc to run it not too sure bout paying greenlight..
i have seen people getting pulled in front of me at the start line because of homemade camera brackets and it was less hassle with scrutineers to remove the camara.
i know if it is secure it should be alright but you should check the green book an the issue