I ported a quick GLSL shader to get bilinear resize (zoom & shrink) on Sprites when you need it outside of the regulard browser resize :
<img src="https://dl.dropbox.com/u/1412774/WebGLBilinearEffectDemo/demo.png" border="0">
You can get some "smoothing" effect when you resize your image with it :
<img src="https://dl.dropbox.com/u/1412774/WebGLBilinearEffectDemo/beforeafter.png" border="0">
The dev page is there : http://www.scirra.com/forum/topic59992_post368714.html#368714, you can now the effect of the various parameter on it.
The demo : https://dl.dropbox.com/u/1412774/WebGLBilinearEffectDemo/index.html