I would doubt any would be denied for this reason.
They would not only have to investigate the claim to determine who worked on the brakes, they would also have to determine the faulty brake work was the cause in fact of the accident. The cost to do that on each and every claim would far surpass what they would save in the one in two-hundred case where the guy who worked on his own brakes didn't know what he was doing.
It would take you, the driver, saying "I pushed the brakes, but, dangit, I must have really ****** them up when I did the brake job yesterday, and that's why I slammed into that other car" in order for it to move a a needle on the insurance adjuster's meter.
Truth be told, the guy working on his own car probably does a better job than the low-level mechanic that gets tasked with doing a brake job. That's the kind of work a big mechanic operation gives to the new guys with little or no experience. It's easy work and it's hard to mess it up.
Do you rotate your own tires? I'd guess more accidents happen due to improperly tightened lug nuts than they do to bad brake jobs.