Thanks guys, some really good input here!
madster: That might be true, but I guess this should still work:
Blur Vertical and Blur Horizontal applied twice (means: Blur Vertical + Blur Vertical + Blur Horizontal + Blur Horizontal) gives me pretty much the result I want. So I guess it should be possible to just write one .fx shader that does exactly that - take blur vertical and blur horizontal twice each and give me a global strength that I can set in the effect globally AND still change during the runtime.
I don't need an exact copy of Photoshops Gaussian Blur, I get something that's good enough with the method described above. The only 'problem', really, is that I have to apply the effects a couple of times and that I can't change the strength of it during the runtime, which sucks if I want to do perspective changes and can't change the blur level that's applied to the canvas object.
Hope that makes sense now