[flopunctro]

I haven't done any research, but my set of anecdata (3 Internet providers from Romania, 2 national and one regional) says that providers do have ways of bypassing their router at Layer3. These ways are not advertised, sometimes not even documented. But they should be just a phone-call away.

If the router is also used as a media-convertor (upstream is Fiber or DSL or coax), they should be able to set it to "bridging mode", where it will function as a Layer2 device (switch), thus allowing the customer to use their own Layer3 device (router).

[RealityVoid]

Most of the times you can set them to bridging mode from the management interface as well.