Hi Brett,
Isn’t it better to let the Pi connect via Wi-Fi to the hotspot of your iPhone? That way, the iPhone shares its cellular with the Pi and can send data. I know iPhone has the ability to use cellular when Wi-Fi is slow/not available, but I don’t think you can send data to your iPhone via the Pi’s Access Point connection and instruct the iPhone to pass it forward via cellular. Changing the flow, so the iPhone becomes the Access Point and the Pi connects to your iPhone, is probably better and achieves what you want without creating an app or something on your iPhone.
About the /etc/dnsmasq.conf, you’ve to run a container with dnsmasq installed and give the container access to the host’s network (network_mode: host), so that the container manages the DNS of a specific interface. This post contains almost everything to help you get started with that!
Edit
I saw that you would like to achieve this for a reason. Only thing I can think of is creating a webserver or something on the RPI and let that act as the interface between your RPI and iPhone. Only thing is, I don’t know how you can force your iPhone to send data via cellular, other than changing your flow.
But nonetheless, I’m very curious if you can find a way to achieve this!