I have an Asus RT-AC66U and put TomatoUSB (Firmware 1.28.0000 MIPSR2-112 K26 USB AIO-64K) on it last weekend.
There's one cosmetic issue thou…
For VLAN tagging (vlans 3 en 4) to work on port 4 (physical port and Ethernet Ports State), port 1 has to be configured in the GUI.
The NVRAM shows the VLANs on port 4 as it should. See below.