Just to clarify: my idea is about a webapp which allows to create/generate custom promocodes, not to use the ones that is possible to require to Google for paid apps. I think this could be a useful service for developers.
As an example, you can create a free app where features and functionalities are unlocked using promocodes (suppose permanently using specific promocodes that you give to a blog reviewer for example so he can have a full app at no cost, or just temporary unlocked as a trial period for common users). When the user decide to buy the paid-version of the app, he can still buy the paid-version from the PlayStore or permanently unlock the free version, directly from the app using IAP apis.
I think that it could help engage more users, also in games apps where user buys virtual goods, because you could post special promocodes on blogs or somewhere else, to give them free goods so to let them be interested in having a look at your blog or somewhere else where you can drive a different way to promote your apps (tips and tricks, promote other apps and so on...).
Do you think it could be a useful service? Would you use it?
Please, leave me a feedback, even a short one but i'd really like to see what people thinks about it.
Thanks in advance