Instead of time-locking these 'secret missions' or your suggestion I think it would be more appropriate to have a weekly mission in addition to the dailies. First, it would allow for 280 - 300 light missions to become more common. Second, it allows players the weekend to attempt to get these rare items. Third, each week doesn't need to have some sort of hidden puzzle. Fourth, it can be a new way to earn 'nightfall' tier rewards.