QinQ sounds interesting, can't clearly tell if it will work.
I'm running Supermicro X7SPA-HF in a M350 chassis,
I haven't see a compatible riser card / IO Panel so a third nic isn't in the cards.
Currently have em0/em1 dedicated to the modems and LAN via a USB adapter which is very dirty.
wallabybob's #2 looks to be the only solid option at this point.