Do i need to be signed in to an Xbox account to enter a particular promotional redemption code?
No. That would be a good hustle for you but Microsoft already knows how that would go for them. You have to be logged into an Xbox Live account(it can be a silver) to purchase items or redeem codes. Those purchases are tied to your account so if you go to a friends and wanna play a movie or video game you bought you can redownload for free to his Xbox under your profile but once you go home and log into your Xbox it wont be able to be watched or played from his box. To make it simpler it goes w the profile.