I was playing with it, and when I did a small image with 2x2 pixels of size, and re-sized it to be a mask (with the transparent area around), but it blurred very much around the image. It maybe an issue if made automatically.
<img src="https://dl.dropbox.com/u/47035927/temp/blur.png" border="0" />
The blue horizontal mask is blurred, but it should appear like the vertical bar. And for tiled background, it will not happen, right?
Instead of using sprites I should used tiled background, but didn't because of the origin points issue of the R100.
Well, it's really hard to resolve, because it's very complex to make the engine place a frame around the images to blur, but if you place the image on the canvas and re-size, it will always blur proportional to the original size, re-sizing the transparent edge too.
It's not possible to make the transparent offsets only on the exported mosaics?
Also, thank you for the information, I'll workaround it by using tiled background if you make it automatic.