I've though about this before, and I think it would be a good idea. Maybe three of the free rotation champs have one of their skins (with limits on which ones) available for the week for free for players to try out. Then at the end of the week the skins go on sale as part of rotation of skin sales. Not every skin sale would be free rotation skins, but some would be. If there is a worry about people over using the free skins, then perhaps there can be some kind of token system, where you get a certain number of free skin uses each week.
Perhaps instead of tying a free skin rotation with the free champ rotation, have the free period be before each sale. This would reduce the amount of time that the skin is free to use, but still give people a chance to use the skin if they wish and then maybe buy.