There is actually a way that users have discovered to play emulated Game Boy Advance games on their iPhone. It’s not always a great idea, because the downloads are considered illegal. Though there are fan-made games out there.
Interestingly, it does not require a jailbreak. The software emulator does this through GitHub and MacBuildServer. Both are used by developers to create apps. However, through these tools, users can install the app on their device without going through the App Store, because they are installing an app that the iPhone thinks is being used for testing and development.
People who have ROMs, or even some fan-made games, can then play them on their devices. Yup, people have done it.