Continuing in my "useless information you never knew you didn't need" series of automotive documentation, I just spent an hour making my eyes bleed staring at Sumitomo's connector inventory to try and track down the plug used by the WRX boost control solenoid. I'm pretty sure at this point that it's the the exact opposite part as the Evo BCS connector: M(6188-0266) for the male connector (the part that's molded into the solenoid itself), and F(6189-0249) for the female plug (attached to the car's wiring harness).
The only difference between this plug and the Evo version is that the little notch (key) is on the left side of the bottom of the plug, rather than the right side. Annoying little gratuitous differences like this are why I hate automakers. :p