I've got an ER-10X now and started to lookup all the datasheets and the GPL code from Ubiquiti to create a port.
The good news is that it's very much the same as the ER-X but the upper 5 ports are connected to an RTL8367RB which is connected to the MediaTek switch via rgmii (1GBit link speed). That integration looks very dirty in the GPL sources and is based on a rtl8367c driver from RealTek that does not exist in OpenWRT yet.
This is my first port so don't expect an image yet or fast progress but if anyone wants to help feel free to do so. https://github.com/decke/openwrt-er10x