I wouldn't expect that to work TBH as a "good" router should not be allowing LAN traffic over the WAN port by default. pfSense for example by default wont accept a private address at all, you have to specifically untick the boxes to tell it to do so (presumably why CG-NAT is supposed to be done on a specific reserved range not normal LAN ranges). From what we've heard of the Nest so far, seems unlikely it has such an option.
Plus I don't like bridging the LAN and WAN anyway as potentially its leaking any broadcast traffic onto the telcos network, though I'd assume its dropped at the other end, its still wasted bandwidth.