You might want to give Wind
http://windmobile.com a look. Basically they have the same plans for prepaid as postpaid. They are only present in the big cities though - no coverage outside of the cities, although you can roam for a fee. They do have data plans. You would be able to sign up for the first month and then abandon it.
Aother negative is that they only use wcdma 1700 frequency, so if your phone doesn't support that you are out of luck. You may want to consider purchasing a cheap used phone. But they will give you a sim if you bring your own compatible phone. I bought mine in the states and activated it here.
Essentially the news is bad. Canada has about the highest mobile phone charges in the world, and consequently the lowest penetration. And I am not aware of the buy it and go sims at the 7-11 like you can get in HK.
I just noticed a Wind SIM on
craigslist. . You may find some other phones there too.