This is not just dynamic properties. NO properties are inherited.
I created a simple test to try to isolate error. Appears to be an issue with the implementation of the <webtest> task.
I found this:
In Ant 1.8 the PropertyHelper class was much reworked and now itself employs a number of helper classes (actually instances of the org.apache.tools.ant.PropertyHelper$Delegate marker interface) to take care of discrete tasks such as property setting, retrieval, parsing, etc. This makes Ant's property handling highly extensible; also of interest is the new propertyhelper task used to manipulate the PropertyHelper and its delegates from the context of the Ant buildfile.
We are having this problem too, though with an old WebTest version, 1762. Is this fixed in a newer WebTest version? We tried to upgrade to the latest version, but some of our html doesn't work with it, we haven't had time to address all the issues. We're working around this by keeping an ant 1.7.1 just to run webtest.