PostFX question, controlling affected area

[__fastcall]

Quick question(s):

Can I apply a shader to a certain area on the screen?  I need to apply a mask over a tile (for every tile) in a breakout-style game.  Is there any significant overhead I should know about when drawing a PostFX?  If I can't apply a shader to certain areas of the screen per tile x60/s, can I use a separate texture, i.e. render target, to render my masks onto?

Thanks for any responses!  
__fastcall

EDIT:
I have tried setting PostFX's scale and position, which rendered the result of the shader (the whole entire screen) onto the area.

[Laurent]

Take a look at the roadmap, everything you need is there and will be implemented in new versions (maybe not 1.4).

[__fastcall]

Ah, so it is not currently possible.  

Thanks, I will research alternative methods.