ng_patch and 802.11Q

[Bear]

Hi all,
I want to modify VLAN priority by ng_patch. After reading the manpage of 
ng_patch(http://www.freebsd.org/cgi/man.cgi?query=ng_patch) and the 
example it given:

 >  /usr/sbin/ngctl -f- <<-SEQ
 > 	 mkpeer ipfw: patch 200 in
 > 	 name ipfw:200 ttl_add
 > 	 msg ttl_add: setconfig { count=1 csum_flags=1 ops=[	 \
 > 		 { mode=2 value=3 length=1 offset=8 } ] }
 >  SEQ
 >  /sbin/ipfw add 150 netgraph 200 ip from any to simplex.remote.net

It seems ng_patch can only modify IP header. However, the position of 
VLAN header is before IP header and after Ethernet header. How can I 
modify it?

Thanks in advanced.