Transparent Wi-Fi module with RMII input
While possible in principle, MII isn't really used for interfacing wireless PHYs, because all the small differences add up.


802.11 has different inter-frame gaps for different traffic classes to give it a crude priority schema
MII only transports user datagrams, so association and authentication would need to be transported over MDIO.
The destination MAC address in 802.11 is the next wireless hop, not the address of the next routing hop, so the packet needs to be rewritten, moving the next routing hop to the first auxiliary address field and adding the AP address
...


So basically the PHY would need to replicate the entire MAC logic anyway, and the effort in implementing the side channel communication would outweigh the savings from reusing the MII interface.

The main reasons to use the integrated MII Ethernet controller would be to use the multicast filter and packet size sorting in memory-constrained devices. The multicast filter needs to be replicated in the PHY, because of the way multicast packets are appended to the beacon, and the packet size sorter is only relevant for very small devices, where tighter integration and smaller footprint are even more important than ease of implementation.