OK, here's how I suggested CJ try his purge system:
First, delete the passive valve and extra hoses that go to the turbo inlet like the v7-v8 JDM EJ207 engines have from the factory. This is how I run since I have the rear-mounted charcoal canister just like a WRX. Works fine for me. The purpose of all the extra junk in the WRX system is to use the partial vacuum in the turbo inlet to vent the charcoal canister under boost, and the purge control solenoid to vent under vacuum. What you're doing by deleting the extra valve and hoses is preventing the engine from ingesting fuel vapor from the canister under boost. The purge control valve will still work as factory and pull fuel vapors from the canister under vacuum.
Then, since he has to have his charcoal canister in the engine bay for now, he needs to interrupt the vent line that goes back to the tank and put a canister in line with it.
Now, you have the purge valve connected to the intake manifold, with the other side of the valve connected to the charcoal canister. The charcoal canister is connected to the tank. If the WRX ECU never opens the purge valve under positive boost (I don't think it does, but haven't tested this theory) you'll be fine as-is with no check valve. If at any time the WRX ECU opens the purge valve under boost, you'll have a boost leak into the canister. The check valve is a safeguard against this and should go in the line between the canister and the purge valve, contrary to how CJ drew the diagram up there. It needs to allow the engine to pull from the canister but not allow boost pressure to enter the canister.
In the older cars that "vent" line is directly connected to the tank. The tank vents to the canister in the engine bay, and the engine pulls vapors from the canister.
In the newer cars, the "vent" line is actually the post-canister line since the canister is at the back of the car. The tank vents to the canister, then the engine pulls from that canister through that "vent" line.
We're just taking the engine that's set up for the new style system and modifying it's purge lines to insert a charcoal canister back in the engine bay like the old setup.