With what you have outlined here, one option to get the values into the unattend file would be just have the powershell script do the substitution in the XML. While LANDESK does handle variables and can do the "substitution", it doesn't work conditionally. You could use a value from the inventory of the device, or a static value, but you can't use one value from the inventory (IP address) to then put a different value in for the variable.
However, variables can also be created at several levels. You can create public variables, which apply to everything, or template variables that override any public variables for a specific template. You can also create Computer variables that are specific to devices. However, it won't do exactly what you are looking for.
thanks for your answer.
In the meantime I wrote a VBscript that in function of the detected IP address firstly creates a reg file and than imports it in the configuration phase of the template.
This way I can change the keyboard layout, currency and timezone settings.