Just an update. Check engine light would still illuminate intermittently with the original ECU.

After e-mailing a couple junkyards that listed matching ECUs on car-part.com with no reply, I just waited for an exact match ECU to pop up on eBay and grabbed that. Since swapping it in, car goes right into closed loop mode and the lambda numbers jump around appropriately. That was a couple weeks ago and the check engine light hasn't come on yet.