How can we pass a dynamically assigned variable from one end of a pipeline to the other?

You define PARAMNAME at the beginning of the pipeline, use passthru where possible, and when you want to use the PARAMNAME later, you use jsonpath to get it.

Other possibility is you get the PARAMNAME value, then use a COPY snap with multiple outputs. Then you use a JOIN snap to bring the value back into the pipeline at other later locations where you need it.