Can a custom filter read property value and use it? the built in filters has some settings, like motion blur has amount. but it' cant be assign with logic bricks.

now a custom filter, can it read a value somehow (python code wont work? or would it... like own["blur_amount"] ? )

the shaders is C code it looks like, and doesn't need to have .py extension AFAIK. 

anyways, this would be so awesome, also for built in filters. so we can enable/disable and maybe script with python the transition.
Yes you can have a game property in a GLSL shader.

Here is a good read on making exactly what you want, a blur filter with a controlled blur amount from a game property (plus blend file).

From the script linked above, you can just use the property name blur_amount in the GLSL script.

Declare the var like this, then use blur_amount however you want to.

uniform float blur_amount;