Parameterization of Assembly Instantiation based on contents of Plan

When you create an assembly by posting to the assembly_factory resource you can pass parameters.
The assembly_factory collection advertises its Parameter definitions. The UI could build out a form based on reading these parameters.

If wish to add instantiation parameters to the POST which are specific to the Plan, you are free to do so but there is no corresponding set of Parameter Definitions on the Plan to guide the UI in creation of the form.

Unless I'm missing something.