Ok, here's the deal. It's actually pretty easy to get out (but kind of a pain). Thanks to all the people who responded and said it could be done fairly easily. That kept me from reaching for the wire cutters.

In a nutshell, here's how the harness comes out of a syncro westy:

1. First, pull the engine (duh).
2. Then, pull the rear seat. It looks really hard to do -- especially when you fill a coffee can with bolts and it still won't move -- but if all else fails, follow bentley: Just pull the two or three bolts holding the box to floor, and the two on the rear deck and the entire seat/storage box pulls out. That is, after you put back the 12 nuts and bolts that hold the box together.
3. Move over to the driver side rear wheel.
4. Pull off the charcoal cannister (just a couple of hose clamps)
5. Unbolt and pull aside the AC receiver dryer. Now you've got all the access you need.
6. In the engine compartment and wheel well, clip about a half dozen wire ties to make sure the harness is "free" and moveable.
7. From inside the van, push the grommet and ecu plug through the firewall hole.
8. From the wheel well, keep pulling the wire harness out until it's free in the wheel well.
9. Untangle it and line it up so you can pull it from the engine compartment.
10. In the engine compartment, feed it from between the firewall and gas tank, being careful not to bend the harness too much (assuming you want to save it...after all, you did go to all this trouble not to cut it), and slowly feed it out into the open compartment.

Hooray. Now, who wants to buy an ECU and harness?
Tim in Seattle.

Thanks to Tim King from Seattle, Washington

Notes on removal:

1. You want to remove everything from the ECU under the seat to the engine. Also remove the idle stabilizer control and its wiring...which, (on my 90 van), was located in the compartment behind the passenger side taillight.
2. You leave the wiring that goes to the AC compressor (and, in the late model vans) there's a wire in this run that also extends to the alternator.
3. Keep the wiring run from the starter to the alternator.
4. You also keep the "van side" of the large round white plug that's inside the black box on the left rear wall of the engine bay....(this plug has the wires that go to the instruments) ...also save the "engine side of that same round white plug and cut the wires coming out of this side of the plug about a foot or so on the rear side of the plug.....you will splce the subaru wires into this plug. (I think others have posted that you could buy a new round white plug...and not cut up the wbx engine harness...in case you want to sell it to someone else).

Thanks to Warren Chapman