I think this might help:
on pineapple, add these iptables rules
-A FORWARD -i br-lan -o eth0 -j ACCEPT
-A FORWARD -i eth0 -o wlan1 -j ACCEPT
on pi:
route add -net 172.16.42.0 netmask 255.255.255.0 gw 172.16.42.1
That way, all traffic from bridge gets sent over eth0, to pi, and pi uses gateway to send stuff back, which gets sent to wlan1 per definition on the 2nd iptables rules.
Can someone validate that i'm not imagining?