@mhgschmidt First, welcome to the community.
This is an interesting approach.
It uses the special case of obtaining a fixed ( static ) IP network when requesting internet access. Similiar to our internal ‘fixed leases’ concept.
Do you have experiences for PPP and DHCP WAN connections?
Maybe we could realize this as a feature in IPFire. Don’t know which parts of IPFire are made for one WAN IP only, if the config isn’t static.