Ok,
Please help me understand what I am missing here.
See attached report (rename .txt to .rpt). I've created a quick sample with ID and Customer Name, added dynamic parameter ID and set Customer Name as a description
Dropped parameter to Report Header - it shows ID
then
Edit Parameter
Scroll down to the last settings of the parameter as shown on my screenshot in my previous post
"Show Value or Description" = Show Description
Now the report shows the Description in the Report Header
Is not that what you are trying to achieve or I misunderstood the problem?