No secret codes according to my mechanic who is also a Subaru tech. He has looked into this during the last wave of US imports when the CDN dollar was at par.
You will have to replace the instrument pod with a metric one as well as a speed sensor on the drivetrain. This will also zero your odometer, that must be reported to the ministry of transport(at least in Ontario), stating reasons why (try explaining that when selling the car). The cost of that about 3-4 years ago was just under 2 grand for parts and labor. The temp control is also part of the LCD module that also houses the odometer which I'm not sure can be replaced separatly, but the same thing applies, zeros the odometer.
I've driven my tech's car which is a US model and you get used to reading the smaller speedo range pretty easy, and as for recording mileage just do the conversion.
I'm in exactly this situation - US production car on the north side of the border and no interest in doing running conversions. It's just not practical - with all the other cars we have being fully metric, it's too much effort to try to remember which one needs the calculation. He11, half the time I'll switch from one of the standards to an automatic and immediately put my foot to the floor where it expects the clutch to be.
As far as $2K to replace the cluster... excuse me while I stop laughing, get up off the floor, and wipe the tears from my eyes. My local self-serve yards charge less than $40. I haven't yet pulled one out of a Forester, but I did with one of my Legacys (a Brighton with manual xmission and no tach - go figure) and it wasn't a big deal. I'm surprised that that the speed sensor has to be swapped as well (the scaling is as easily done in the electronics), but I'm glad you mentioned it.
And that the replacement odo will read zero presumes that you're getting a new one rather than used - in the latter case you get what the donor car had, and responsibly recording the various odos' numbers is an exercise left to the reader.
As for the legalities... DILLIGAS? The more you work on your car, the less meaningful the odometer reading is. I've replaced both the motor and transmission in my '95 Legacy. Someone can tell the body condition (rustbucket!) by looking at it. So what good is the odo beyond telling you how many miles are on the front bearings, rear diff, and wiper motor? Not much. I have no intention of selling any of these cars, but if I were to I'd readily disclose the "true body mileage" without being asked, in the case of a cluster replacement. I don't know what the government's rules are and really don't care. And the "reason"? How about "old one busted"?