I don't know how to do this, or even if it can be done.

I've an RB532 connecting a wireless network to an adsl modem by ethernet.

I need to connect a local PC to the Internet with pppoe for AAA, but there are no free ethernet ports on the 532.

I can connect the PC to the 532 via the modem, but can't see how to set up a PPPoE tunnel between them as the PC (or at least the pppoe client interface) must be on the same network as the modem and the 532: ie

modem: 192.168.1.1/24
RB532 192.168.1.2/24
PC-pppoe 192.168.1.3/24
RB532-pppoe 192.168.1.4/24

If you put the pppoe interfaces on any other sub-net the modem will treat it as 0.0.0.0/0 and route it out of the public interface. Won't it?

There is a static-routing facility on the modem. If I put the pppoe interfaces on a 192.168.2.0/24 network and static-routed that out of 192.168.1.1 would the pppoe tunnel from the PC through the modem to the 532, and then be returned as Internet traffic through the modem with a masqueraded 192.168.1.2 IP address? And the replies dst-natted back to 192.168.2.1 (the PC's pppoe interface) through the modem?

This is where by brain loses its grip on reality and starts singing La Marseillaise in a loud voice with its face to the wall. Rescue would be much appreciated.

pppoe is layer2 so you can't route it. You have to use bridge.

pppoe is layer2 so you can't route it. You have to use bridge.

In other words it can't be done. The PC is running Windows ME (with RASPPPoE available) and the LAN side of the modem is a simple switch, if that. Bridging such a setup is not possible.