The reason why the GC API in is not completely implemented in RAP is that some features are hard or impossible to implement in all browsers. For example, in IE7 and IE8, the webclient has to resort to VML to draw, which is very limited. But also the HTML5 Canvas API does not cover all features of the GC API. I guess that transformations are only possible with SVG, which has already been considered as an alternative to HTML Canvas.
I'd suggest that you open an enhancement request for your requirements.