While it is possible to configure P1 and P2 for different netowrks, can an additional default route be configured based on the client IP address? This would be similar to policy-based routing. So far, it seems that a combination of ip spoofing and PBR on upstream routers will will do the trick, but it seems to complex.
Only a single default route can be configured. This means that all traffic leaving the device will follow that default route as a means of last resort. If P1 is the default route to the Internet and P2 is also accepting traffic for proxy connections that traffic on P2 will still leave via P1 to reach the Internet.