I too have thought of doing this but my consern is that the water ph value could mess up the results.

This will work, though the pH of the water, as suggested, should be 7 prior to adding the soil.

I test my own soil all the time. Use distilled water and don't worry too much about the proportion of soil to water since pH is a logrithmic value. The only thing is that the experts suggest you run a buffered pH test to tell you precisely how much lime to add. I suggest you play with figures,; don't add more than 3,000 lb/acre lime/yr. and check pH the following year. To be on safe side use a mix of calcitic and dolomitic(High Magnesium)limestones.