Other major platforms (namely Facebook and Twitter) make it incredibly easy to create a Follow or Like button for a given resource. I think it would be a good idea for Twitch to follow suit and allow Follow buttons to be created using simple copy-paste codes.
I have written a beefy application and have no trouble integrating a programmatic Follow button using the API, but I’ve given it some thought, and I really don’t want to ask the user for any ‘edit’ permissions when authenticating. With all the shady behavior that’s gone on in the community (phantoml0rd, TmarTn, among others), I’d like to advertise the fact that our application will never modify user account data directly.
In the case that the Follow event couldn’t be completed directly in the embedded context (whether for security or technical reasons), a sized popup could be launched allowing the user to complete the action directly on the Twitch domain. This is how Twitter handles events in their embedded timelines.
A search on the dev forums for this topic yields about 10 results posted by people looking for a similar solution.