Well you could but it won't necessarily work so well, the issue is that you need to keep the total download below your 22Mbps or whatever it is. To do that you can do the following:
- create a veth0 - veth1 pair (requires you to install the veth packages on your router and maybe the full "ip" suite)
- put veth1 into your br-lan bridge
- alter your routing table so that all packets coming in on WAN get forwarded via veth0, which will send them down a virtual wire to veth1 and inject them into the bridge where they'll get distributed between wired and wireless
- place your layer_cake SQM on the veth0 device uplink/egress
You can create the veth pair as follows in your firewall.user script (untested)
ip link add type veth
ip link set up veth0
ip link set up veth1
ip link set veth1 master br-lan
then you'll need to add a static route so that all packets coming in WAN destined for LAN are sent to veth0 rather than br-lan, and put an SQM on veth0
note that this is a little tricky, you could lose your connection to your router if you route to veth0 and it's not properly got veth1 in the bridge or something like that. Save your config backup before you try this stuff, and be prepared to revert via failsafe.