Trying to install OpenWrt on a Velocloud 520-AC

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.

it's really too bad because it's not worth anything anymore

That's actually a really good price too. I might buy this just because it's so cheap lol. I mean like I said, if you're willing to use the SFP ports with rj45 adapters it's really not a bad little router.

Also the wifi hardware in these is pretty robust considering this isn't really meant to be an AP.