Your argument seems to be that:
1. All that bolsters up the system is moral.
2. Some lies bolster up the system.
Therefore, 3, some lying is moral.
The argument seems valid (the conclusion follows from the premises). But what about the premises? Let's look at premise 1. Would the truth of that premise not depend on (1) the particular system? Suppose the system were the system of government in Germany in the 1930 and the first half of the next decade. We might say that anything that bolstered up the Nazi system was immoral. Then, look at premise 2. Kant's argument is that there are no "good" lies, because lying in general, and in the long run, undermines truthfulness, and that undermines the society.(2) If everyone thought it proper to lie whenever he believed the lie would have good consequences, than no one would ever believe anyone else. (3) And it would make no sense ever to tell the truth, because no one would believe you.
(1) Nothing was said about political system, the post was about the general tendency of society and a person's place in it as a social animal. The Nazi's, The Khmer Rouge, the happy happy unicorn lover's nation, doesn't really matter, the basic interaction of human to human and the participating in the survival of the species in group is the point. It was a post based from a socio-biological base, not a political one. the beauty of arguments like that is that they describe human behavior not logic.
Also if I were talking about a political system, the morality of the system was not in the original question. It still stands that people within the system no matter how corrupt are still people with the same basic needs to trust one another for their core social wellbeing. The overarching political ideology of a nation has little to do with the day to day personal interaction of its people. If a person's lie upheld the Nazi political system but still provided inner stability to that system, then the liar and his/her peers would still be living in a more stable social setting.
(2) The post had no mention of intent. It had mention of the result. Aside from this most of us have an ingrained knowledge of what is an acceptable lie in our social system and what is not. It is built into the larger cultural system. If a lie or an option for a lie is built into the system, it cannot really be immoral as the lie is expected. Again see number one for "system"
(3) Again the rationality of a system and the practice of its participants are not the same thing. We believe people because we want to, even when we know they are lying in many cases it does not make us distrust them on a fundamental level. We distrust people when their actions, words, lies etc... hurt us and ours or do perceptable damage or threaten to do perceptable damage to the social system. Say you have a fundamentally good child that you catch in a small lie, you really don't distrust that child because of the lie, s/he may get punished for continuity's sake, but your fundamental attitude is unchanged.