All building codes that I've seen prohibit "solid fueled appliances" in garages. That's a fact.
Now for the difficult part. What's a solid fueled appliance? The codes are not clear, but in general any solid fuel appliance with a direct open door path to a flame would be prohibited. The code does allow an appliance manufacture to certify where a "solid fueled appliance" can be used. So if your particular appliance is garage certified in could be used and not in violation of the law. My opinion is coal boilers would be more likely be certified than coal stoves. The code does also allow for "an engineered" solution to the safety hazard. This is where an installation would have a Professional Engineers seal on the design drawings. In effect this is transferring the design risk to the P.E. A third method is approval of the "Authority Having Jurisdiction". This is usually your state's fire marshal. Good luck trying this.
Now what does an insurance company have to do with this? Nothing, absolutely nothing. They have no authority to enforce laws. They can't waive any law requirement. What they do is insure risk. They look at your installation and judge if you are a reasonable risk. In principal, higher risk, higher premium. Too high of a risk, for one insurance companies underwriting polices results in no coverage. A insurance company will insure risky things, even stupid things for the right premium.