Hmm, I think the biggest problem with using the front cover is that the routing of the line will have to make a tight turn at the cover, which is not good for flow. Also if your mounting the turbo at the other end of the engine, as most people do, since there is space there, the line will be long, also making flow capability reduced. The return has to be on a downward angle from the turbo to the point where it attaches to the engine, if there are any low points, the returning frothy oil (the oil turns to froth in the bearing housing of the turbo), will not drain and you will experiance a blockage and blow oil through the seals out into the compressor housing and/or turbine housing. The oil drain back also has to be quite large, since as mentioned the oil will turn to froth and also has no real pressure , so a large return that only flows down will help with gatting the froth back into the engine where it will turn from the frothy state back into a liquid state. This is also where turbo mounting position is important, as high as feasable in the engine bay, to help promote drain back.