Is it possible to nest build parameter names in a build config?

I have the following situation:

Configuration parameters
variableName.env1 = someValue
variableName.env2 = someOtherValue

System variables
system.env = {prompt: env1,env2}
system.variableName = %variableName.%system.env%%

In other words, I want to use system.env (prompt at runtime) as a key to lookup the proper value of variableName, so that if the user chose "env1" then system.variableName would be "someValue".  The syntax I just used does not work, but is there a way to do it?  (This is a Java Ant build, btw.)

Thanks for any help.
Mark