Melatonin was somewhat helpful for me with regular opi detoxing. I am not sure if it would be more or less helpful with a suboxone detox. From everything I have heard, some folks find it helpful, while others don't get any benefit from it. It is most helpful in getting your internal 'clock' reset though (i.e. falling asleep at the appropriate time). I am not sure if it would be helpful for staying asleep, but it might be worth a try.
By the way, be sure to check the active ingredients of other OTC sleep meds before trying them. Many of them use the same ingredient that is in Benadryl. For example, many of the unisom tablets use Diphenhydramine HCl in doses equivalent to 2 Benadryl tablets. There are other unisom products that use Doxylamine Succinate, which blocks histamine and acetylcholine. The Diphenhydramine HCl works only on histamine from everything I have seen.
Unfortunately, I don't have much other experience to share with OTC sleep aids. I wish I could be more helpful.