You have regional BGP routers that route subnets to
the internet. These could
then tunnel the subnets to end users via GRE. End users could route via an
IGP over this tunnel to the regional speaker(s). Multiple tunnels would give
redundancy.
Not throwing stones here but we all saw what happen when URO went down a few years ago.
This topology did not work well for us, I would have seen Hub0 as a regional routers.
People lives change all the time so keeping it simple work well and has worked well for a
lot of years.
I for one would prefer to route my own Gateway even if I have to deal with a little
latency.
Jerry, kd4yal