The FreeBSD man page for lagg gives an example of configuring a lagg with a WiFi interface as a member.
The pfSense lagg creation page says Only unassigned interfaces can be added to LAGG. which would suggest pfSense won't allow wireless members of a lagg (because a wireless member would need some configuration which would require it to be assigned to pfSense and assigned interfaces can't be lagg members.)
In practice though, things are different. An attempt to create a lagg on one of my pfSense systems offered run0 as an interface even though it was assigned to pfSense as WLAN and didn't offer ral0_wlan1 which was assigned to pfSense as OPT2.
It is not clear why the restriction Only unassigned interfaces can be added to LAGG. is documented, especially when it appears inconsistently applied (it appears the restriction might apply to wired interfaces and only SOME wireless interfaces). The restriction prevents creation of a lagg capable of transferring jumbo frames (the lagg members need to be assigned to pfSense to change the MTU but that appears to make them ineligible to be lagg members.)
Suggestion: remove the restriction and fix the other quirks relating to lagg membership documented in http://forum.pfsense.org/index.php/topic,50563.msg269325.html