Closing.
Brief:
1. Access to an internal buffer is not an option.
2. Pushing data back to stream after fetching is not an option too: it kills almost any possible optimization and makes code overcomplicated. aiohttp used to have "unread_data()" API but we deprecated it and going to remove the method entirely. A wrapper around the stream with puback functionality is an option though unless it doesn't touch underlying stream implementation.
3. Extending a set of reader operations is a good idea, please make a separate issue with a concrete proposal if needed.