From the airshow's website: "Colorado Sport Aviation, Inc. (CSA) is an all-volunteer, 501(c)(3) non-profit corporation dedicated to serve and promote recreational aviation by hosting an annual Air Show and Regional Fly-In."
From FAR/AIM (Federal Aviation Regulations / Aeronautical Information Manual: The term "firearm" is mentioned many times, but a thorough review revealed in all instance it referred to firearms being carried in flight. No mention was made anywhere with respect simply being armed while on an airfield.
TSA Regulations only require that you not bring firearms to the security screening area. There are no restrictions of the carrying of firearms prior to that point. In DEN, you can concealed carry, as Denver County extends to and includes the airport, and at COS you can carry any way you wish. I've OCd at COS. Got some funny looks, but it's legal.
A related issue involves Hartsfield's (Atlanta, GA) request to the TSA to modify it's security program so as to ban firearms anywhere on airfield property, including parking lots and access roads. Currently, no airport can modify its security program without TSA approval. GeorgiaCarry.org, a gun rights group, sued. GeorgiaCarry.org lawyer John Monroe said the airport gun ban jeopardizes personal safety. Click here for more on that story.
As a worldwide military aviator, I've carried firearms all over the place. Some might think we have a carte blanch waiver because we're military, but that's not the case. We have to follow local laws, as well. Usually we just locked them in the on-board gun box before departing the aircraft. Since our aircraft are considered sovereign territory, the same as embassy grounds, we can carry on board all over the world, but departing the aircraft while carrying them was usually a no-no, unless we were either at a military airfield, such as Pope AFB here in the states, or one under military control or authority, such as most of the airfields in Iraq.
Having reviewed the applicable regulations and information, unless they have a TSA-operated passenger screening area, I do not believe their "No Firearms" mandate is legal under Colorado State Law. I believe this falls into the same category as the Colorado State Fair's illegal mandate, which I covered in excruciating detail, here. I believe many, if not most, of the same arguments apply.
Bottom line: If it's public land, and not protected by federal law i.e. behind a TSA-operated passenger screening area, State law applies, and State law says we can both OC or CC with a CHP. Our choice. They might argue that because it's run by CSA (Colorado Sport Aviation, Inc.) that CSA gets to make the rules, but they'd be wrong. It remains public land, and Jefferson County cannot encroach on the rights and freedoms of people just because it either hires or allows an organization to run a business on public property.
Question is: Who's going to sue these two un-Constitutional organizations?
Please feel free to use what little information I added in your letters to these folks. Don't expect them to back down, however. They'll stick to their guns (or lack thereof) until they're forced to do otherwise. A lawsuit might work, but a preemptive injunction by a judge would carry more weight.
Another option would be to coordinate with the local law enforcement having jurisdiction over the airfield, presumably Jefferson County Sheriff's office, obtain their buy-in, and simply show up armed with their pre-briefed officers backing you up as you enter the gates.
Good luck on getting their buy-in, though...