The monowall kernel is small and dont support a lot of different wlan cards. Also you need a wlan card with hostap support.
any wlan card with atheros chip will work with m0n0wall. it is little expensive in compare of an access point.
but then i moved to use separate AP. I guess there will be no need for WEB, WPA2 as long as there is a captive portal.
If m0n0wall support running captive portal on bridge interface if_bridge. m0n0wall will make a very good access point. and less expensive, much much less.
regards.