I'm using 192.168.0.0/24 (but that's OK obviously). DHCP is off (I prefer static IPs). There's no way to specify independent IP ranges for the LAN and WLAN in the settings.
However, this isn't a WLAN / LAN / IP / Subnet type issue - it's an ARP Broadcast issue. It wouldn't matter what IP or subnet I was using because the broadcasts are sent to FF:FF:FF:FF:FF:FF, so they hit every device on the same network segment. As I only have one router the entire LAN/WLAN is the same segment. The only thing my router is blocking broadcasts from/to is the WAN.
The only way to prevent broadcasts spreading too far is to add a segment to the network by adding another router as routers don't normally allow broadcasts to pass through. I could, for example, add another WiFi Access Point router and put all my mobile devices on that. But again that's additional hardware that I'd like to avoid.