That's what I referred to with 'possible, but a heck of a lot of work', sources are available, but they are very invasively changing igb (with the necessary MDIO communication channel for the marvell switch). Getting this extracted from the ancient kernel sources and then forward ported to OpenWrt's modern kernel will be... difficult... and time intensive.... and given the effort, it would have to get merged mainline first, which makes it yet a notch more difficult.
It's a 'possible' task for someone experienced, but expect it to take months+ of rather constant development and bartering with Intel, Marvell and linux-netdev, assuming you find this hypothetical someone in the first place.