I believe you could create a sky loop by doing some post render editing in a movie editing program. As long as the editing program allows you to "cross dissolve" film clips. What you want to do basically is dissolve the end into the beginning. This should effectively dampen the abrupt jump from start to finish. Let's say you have a good ten seconds of sky animation. What you have to do is use two clips of the same scene. Using the editing program of your choice, insert the first clip, but start it from 5 seconds in. Next, lay in the other clip overlapping the first clip by a few seconds. Note: you may have to select a "dissolve effect" depending on the app you are using. Next, chop the last five seconds off the second clip. Render your movie and you should have a smooth transitional loop of the sky slightly longer than your original clip. Experiment with the length of the dissolve to see what works best. It's not the most elegant solution but it should work. Good luck with it.