You will need the RSS feeds for each of the branches you want to show commit activity on and merge them.
The RSS feed for your current default branch is as follows:
https://github.com/{username}/{repo-name}/commits.atom
For each of your other branches (including "master" if that's not your default branch view) they will follow the URL format of:
https://github.com/{username}/{repo-name}/commits/{branch-name}.atom
You can work out what branches exist in an repo by hitting the API with the following:
https://api.github.com/repos/{username}/{repo-name}/branches
The JSON response will list all the branches and the last commit made to each branch.
Do what you need to do to collect them all and poke them through a single filtered view.