Remember,
You need to have a metallic path in order for anything to happen in the first place. If you are not sure of this take a resistivity measurement between the waterblock and the radiator. Also, as you can see on the chart in the above link, the two metals are not that far apart. In short, the potential difference between the two metals is not very substantial so any problems should be minor. One other thing going for you is the use of distilled water. DW has a relatively high resistivity which will further inhibit the corrosion rate.
If you are still concerned about this, you could electrically isolate the radiator from the waterblock. Of course this may not be an option depending on how it is installed. Perhaps the thing to do would be to take it apart after a month and give it a good ol inspection.
edit - guess I should add that the above info primarily pertains to the presence of corrosion between disimiler metals.