No steels are available aftermarket. At first I heard the spare won't clear the front brakes, then I read someone tried it and it did. So, buy 3 more spare wheels from Subie. Or if their price is ridiculous (likely), get the cheapest alloys you can find (I've heard $130)
The problem with using the steel spare wheel is that there aren't any 17" tires available that will fit a 4" wide rim except the temporary use tire which I expect would be worthless in the snow. The WRC cars use narrow 135 series snow tires, but those are for 15" wheels. On gravel and snow rallies they use smaller brakes then on tarmac. That lets them use the 15" wheels on those rallies. The coefficient of friction is so low on gravel and snow that the big tarmac brakes are overkill and just add unsprung rotating weight.
What you guys really need is the center of the STi spare wheel welded into a wider steel rim. (My last F150 had a full size spare on a 7x17" steel wheel....) Maybe a custom wheel shop? Then you could use something like a 205/50R-17 snow tire. Those are the narrowest 17" snow tires I found and need at least a 6" wide wheel which is why the 4" wide spare wheel just won't work.
I'm just glad I live in Arizona where it only snows about once a year! It melts when it hits the ground too!