Accessing non animateable properties in a constraint?

Hi,

I'm trying to find a way to access non animateable properties in a relation
constraint. 
In my case, the property I'm trying to access is the "specific
distance" property in the "depth of field" tab of a camera. This
property needs to be driven in real time by a midi controller so no matter the
workaround, it needs to be flexible enough for me to input what I want into the
property.

Any ideas? Oh and unfortunately I have a very limited knowledge of Python so
please give me a little more than a pointer :)

Thanks,