cool idea. i'd thought about putting the catch can between the aos and the engine, but there are multiple inputs to the aos so would be a nightmare. if you were going to go to such extremes then go to an aftermarket aos altogether (lots out there for subis):
Air/Oil Separator - WRX/STi/LGT/FXT - GrimmSpeed
getting at the actual aos might be a good way to do it. here's a link to a thread with pics of one cut open:
AOS DIY 997.1 C2 - Rennlist Discussion Forums
should be possible to tap the chamber somehow. or perhaps remove the top diaphragm altogether, tap the opening and somehow route to an aftermarket aos?
or ... why dump it into the intake at all? intake is just used for: vacuum, emissions (so the vapour doesn't go to atmosphere) and a place for the oil to go so that you don't grease up the road. so how about an electric vacuum pump? this would provide consistent vacuum, and it could vent into a catch can. I had looked into this; there is some talk online about using a secondary air injection pump. I think the issue is duty cycle - it would have to run 100% of the time, and pump oil and not just air.
and the final note is that the boxster aos has gone through several iterations (check the last digit on the part number) - make sure you get the latest one for your car.