So, I have made a little progress toward getting this to work.
I added an alias to br-lan by adding the following to /etc/config/network
config interface wan3
option ifname br-lan
option proto dhcp
and added some firewall options based on the other interfaces that the Mk5 can receive an internet connection from (wlan1, usb0)
/etc/config/firewall
config zone
option name wan3
option network 'wan3'
option input ACCEPT
option output ACCEPT
option forward ACCEPT
option masq 1
option mtu_fix 1
config forwarding
option src lan
option dest wan3
config forwarding
option src wan3
option dest lan
What this leaves me with is:
1. When I boot the Mk5 without the Ethernet cable plugged into a router, I can connect via WiFi on wlan0 and get a 172.16.42.* address.
2. Plugging into a router and issuing "ifup wan3" adds a second IP acquired from the router to br-lan. This can be seen by issuing "ip addr" (if you have ip installed)
3. The original device that connected via WiFi can now access the pineapple directly at 172.16.42.1, and connect to the internet, and the Mk5 can be reached over the router's lan.
4. Any subsequent devices that connect revert back to the original behavior of being bridged through the Mk5 and talking directly to the router for dhcp assignment.
My current line of thought is setting up a custom route for the 172.16.42.0 block of addresses.
Any input toward getting this working is appreciated.