Hey, posting for posterity since the OpposedForces and Wheeling Subaru links are both dead, the part number is 73711-FA000 and supersedes 73710-FA020. Here it is on Subaru's official part website:
Part, Diagram, Diagram (alternate in case above breaks)
I'm not sure what this thing does, but it doesn't appear to be on a vacuum routing diagram. I don't have one under my hood because the car's been fiddled with so much, but from a quick search it's not acknowledged anywhere on the diagrams I could find. Initially thought it was part of pulsed Secondary Air on California-only cars due to a diagram pointing close to it, but the solenoid shown on manufacturer data has a little filter on the end of it like an EVAP Purge Sol.
Hope this helps anyone in the future as this is currently the top result on Google when looking for an intake/vacuum valve on a GC with an EJ18.