So is it possible to prototype with bridged networking with the virtualization frame work at all? Or is the entitlement required? Even code signing with the entitlement locally just immediately results in a killed process, root or otherwise. It doesn't seem possible to even request the entitlement without paying to enroll in the developer program, sometimes you just want to test if something is possible locally....

Indeed. I encourage you to prototype your product using that feature. VZVirtualMachineConfiguration.validate will fail without com.apple.vm.networking entitlement when attempting to setup a VZBridgedNetworkDeviceAttachment for a guest OS even when running as root. It does not seem possible to prototype without the entitlement.