Simple: These rogue nations have no need to do anything differently than they have been doing.
Santa only goes to the homes of those who not only celebrate Christmas, but have also managed to get a letter sent to the North Pole for the yearly Christmas Registry.
In rogue nations, there's no need to do a true "cover-up" because if a household writes a letter to Santa, the secret police will round them up and imprison them such is the case for North Korea. In the case of Iran, the concept of Christmas being celebrated is still so new that the details of how the holiday works (such as the letter-writing) are still largely unknown that people don't do it. If anyone tried, it'd be easy for the Iranian government to filter out the letters from the international mail supply.
Santa won't go down every chimney because he has no idea if there are children in that household and he has no way of knowing if those kids were naughty or nice since he hasn't had an opportunity to do research on them. If he were to enter a house uninvited, it would then open up sanctions against his company alongside a warrant for his arrest for trespassing, so he is very careful not to cross any boundaries lest the other kids of the world miss out because of his mistakes.
As for how major world powers could be prevented from full disclosure, the trick is to treat Santa just as they do the aliens. Only a select few from each government knows the truth of Santa's existence and those who aren't informed are kept in the dark and fed time-tested lies to make sure that they don't learn the truth. Once a person goes into political office, they are required to fill out a legally-binding contract where once they leave office, they cannot share with the world what they have learned as a matter of international security.
Nations like the United States of America were required to sign into practice laws such as section 1021 of the National Defense Authorization Act for the 2012 Fiscal Year which allows the indefinite detention of anyone who is suspected of terrorist acts without trial or a lawyer. While citing the war on terror as the reason for the need of these bills, in truth, it's to keep the secret secure without needing to do a large-scale organized cover-up. After all, if the world already thinks they know the truth, are you covering it up by keeping those who wish to disturb the world order from acting haphazardly with their knowledge? No, it's not a cover-up at all; it's peace-keeping!
As for preventing Santa's tech from being leaked publicly or being used by the governments, Santa's company is unofficially recognized by the UN as one of the 196 nations of the world, but Santa's Workshop doesn't send a representative as he prefers to keep out of world affairs. In turn, the UN doesn't formally state their recognition for the aforementioned points of international security. As a result, Santa's workshop cannot be conquered without UN provisions demanding other nations to go in and protect that country which will be treated as a Canadian town for the sake of paperwork. As for why Canada doesn't take the opportunity to invade: it'd be hard to convince the Canadian military to "invade" their own territory. They'd question the orders if not outright refused due to the sheer lunacy of the matter. If the government claimed that the town was a terrorist sleeper-cell colony to convince the military to act, then Santa would be forced to turn his technology against the Canadians. Since Canada's military consists of Mounties wielding pitchers of maple syrup, the Canadian government realizes it's better not to antagonize Santa in case they need to rely on his technology to protect the Canadian people.
In the end, Santa is a tactical genius who placed his company in America's hat where nobody would invade to take his cutting edge technology. If somebody tried, it'd be treated as an attack on Canada and Canada's allies would defend it, alongside the full force the UN could gather. This means America would send troops to fight in defense of the workshop as well. For the countries that wish to keep Santa from visiting their people, they just stop their nations out-bound mail from going to the North Pole. This way if Santa visits any households in their borders, they can try him as a criminal, so Santa steers clear since he wasn't given an explicit invitation by any of those homes. In turn his company sends extra gifts to the families of those with clearance who are holding political office in each nation as a way of thanking them for not interfering with his company. One of those gifts was giving the internet to the world leaders before the public got it in order to facilitate communication and peace between nations and their families.
