If you have an wireless link to an AP with mode=station and no WDS it's almost imposible.
Ok, given, I have not actually tested this, but theoretically, I see no reason for it not to work. If you actually used WDS, then I might understand it not working.
You have a AP without a IP (I hope), to terminate PPPoE On, let's call it wlan1 You have a 'backhaul' connection, let's presume also Wireless, connecting to your back-end network, let's call it wlan2, IP 1.1.1.1
You have a dedicated access concentrator in a office, IP 2.2.2.2 For simplicity, we are presuming that the network is operational, fully routed, and working.
On the high site, a bridge is created between wlan1 and wlan2 - let's call this bridge1. Now, PPPoE coming from the client connected to wlan1, will also be available on wlan2. wlan2 *does* have a IP address, 1.1.1.1
From 2.2.2.2, we create a EoIP tunnel to 1.1.1.1, because of the tunnel, PPPoE will also show up at 2.2.2.2. You install your PPPoE Server on 2.2.2.2, listen on the EoIP interface, and all your connections will terminate there.
Sure, this is highly over simplified, you need to have some really funky firewalls here - ideally you want to ensure that *only* PPPoE goes over the bridge, and the EoIP tunnel. But untill I try this myself, I see no reason for this not to work...