Officially: It isn't possible with the current shipping version of Hype to keyframe on transform:scale(). Your workaround is the best method if you want to use it.
Unofficially: I'm working on this today . We planned to make it part of 3.0, but ran out of time to finish. Your animation as it is will also be improved because we'll be using fractional values for position/size in the next version. (Of course we haven't shipped, there could be issues requiring us to pull this. I also can't comment on a release schedule).