Hi Steve.
I don't know this feature. Default values can be defined within your JScript function using undefined parameters analysis, but this will not help to get the behaviour of TC you described.
I can suggest to create fake KDT as set of templates: collect all the function calls you need, populate all the "default" values for parameters manually and then copy-paste function calls from this KDT into your real KDTs. This approach may look not handy enough but you may create different templates (set of parameters values) for the same JScript function.
