Have you tried the performance paddock and hot rod forums?
You don't want it to drain upwards because you're putting an additional hindrance on the pressure feeding the turbo. Plus you'll coke the oil that will inevitably sit in the turbo when you turn off the engine.
There might be sufficient flow and pressure from the port where older M116/7 engines feed the mechanical oil pressure gauge.
I guess I don't know how a 420SEL exhaust system looks. In the 450SEL I used to have, the exhaust pipes didn't come together until after the transmission. The exhaust charge would be pretty cool at that point. Ideally, you want the exhaust charge right out of the head so it's packing as much energy (heat) as it can.
See if the folks at www.forcedperformance.net
have worked on any M116/7 engines.