Hi,
welcome to the IPFire community. 
This is exactly the right place to ask such questions, and I’m glad that blog post was helpful to you.
Sadly, I am not very familiar with Apple devices and their proxy configuration/provisioning methods. There is this thread, which goes a bit deeper into this topic, but it does not look like there is an easy one-click solution to this, if I understood it correctly.
If you have permanent control over these road warriors, manually configuring the proxy would be an option. But I assume this is not the case here…
In corporate environments, Apple devices can be exhaustively provisioned by an MDM (mobile device management), but I have no experience with that, either.
Sorry to disappoint, and best regards,
Peter Müller