Our club would allow about anything safe so long as the gun is too powerful and the shooter recognizes that if his equipment or style deviates too much from the club standards in way that gave him an advantage others couldn't have and still be "legal" he wouldn't be eligible for the "win" in the class he is trying to compete.
For years we had a guy compete with us that used a bipod from the prone position mostly, but also ranged and clicked normally (I think) and shot in open class. Since he normally wasn't a contender for the win, it wasn't an issue. his scores were posted and nobody quibbled, they knew he was practicing the way he truly hunted, and so it was no biggie.
Say, for example a guy wanted to shoot in the "International class", but used a harness ... well, he couldn't claim a win if he had high score in the class, but he could still SAY he was shooting in that class if he did all the other stuff required ... just would miss out on a chance for a trophy