It sounds like you have a public address pool and you want to assign these address to VM interfaces. Perhaps then you don't need tenant routers at all?

It is a legitimate request, and it should be possible to boot directly a VM on an external network. However only admins are allowed to do this (or to be more precise the tenant which owns the external network). I will check on the issue you reported. I assume you are using Neutron with the OVS plugin?

No-NAT routers can be considered too, but implementing your use case would be trickier, and I would recommend more directly booting instance on external networks.

The last thing you might want to consider is a provider network. It does not have to be external and you can use the provider extensions to map that network to the physical network (possibly specifying a VLAN id as well) connected to the internet.

You need to boot the vm with the --nic net_id=UUID_OF_PUBLIC_NET and then it will get plugged directly in. Your hypervisor needs access to that provider network of course.