It's common in the datacenter to come across motherboards with a switched eth0, with the BMC behind one leg and the user system behind another. You don't have to get that creative to get IPMI out of a machine when the OS is hosed -- to be honest, I think that is what you're actually thinking of, because "hook[ing] in on ring 0" is difficult to imagine working. You'd need driver awareness for when the management plane wants to transmit, at the least.