Hi Dan,
You will need two project variables - one for the variable part of the address and another one for the full address. Specify the second variable in the Namemapping.
Set the required value to the first variable and, at the beginning of your test, execute a code that will assign a proper value to the second variable. Your namemapping should work after that.
Note, that as these variables are of project level, they will be accessible in all (keyword) tests of this given project.
You may implement the same logic on the project suite level as well if required.