Is your classpath in your properties file complete or do you need to modify it? Or build it in the build file?
I'm not really sure what you need to accomplish, but you wanted an example using path convert tag. I used it with a copy task to take a path that was specified and "build-converted" in the build.xml (maybe you are wanting to just get it from the properties file?) and put it into a .bat file.
Your mileage may vary, I was using 1.6.2. I used the example code for Copy and PathConvert from Ant Manual.