I've done exactly that, when I couldn't be arsed to figure out QinQ on a wrt54g which already uses VLAN tagging internally between the SOC and the switch ASIC.

I've done exactly that, when I couldn't be arsed to figure out QinQ on a wrt54g which already uses VLAN tagging internally between the SOC and the switch ASIC. Just used two physical ports and a splitting scheme just like the above. The trick would be doing it with _one_ physical port on a suitably malleable GigE-equipped device.