Unfortunately that won't work. Any remote fastboot commands get denied.
With the previous OS (and I'm speaking mostly for the Dream, at the moment, because I was able to root a Magic w/o any troubles.) you needed to install that Flash Recovery app and it would work. (Step 3 of that post you linked to.) It was a KEY step.
I installed it and the recovery flash app didn't work.
However I haggled Rogers into giving me the Magic upgrade even though I wasn't eligible for it. (I threatened to cancel, which btw, I was sooooo ready to do).
I'll have to stay on their backs to make sure it gets delivered to me once they have stock but once I have a backup phone, I'm going to mess around more with this current one. I'm sure someone at xda will beat me to it though!