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

Hey Scott,
As mentioned already, there are several complex options that can be used as a workaround. Those mentioned already are as good as any, so I refrain from mentioning more. Also, I agree with Patrick, that it may behoove your organization to reconsider their position on the Pipeline Execute snap.

However, based solely on your example of needing a dynamically generated date stamp constant throughout your pipeline, this may be a less clunky option for you:

Try using pipe.startTime (or a formatted derivative) as part of your filename expression.
"test_" + pipe.startTime.toLocaleDateTimeString({"format":"yyyyMMdd"}) + ".csv"
I believe pipe.startTime should remain constant throughout the pipeline run and this might provide what you can use in your specific use case example.

This won’t solve other pass through scenarios though; which is basically the meaning of the title of your post. However, another option for those scenarios might be to place your dynamic expression in a pipeline parameter and use the eval() function to evaluate the parameter. I have used this technique before. As long as it is not an expression result that will change with run time, it should work.