Dana, you can do what you want but not exactly the way you're thinking. Parameters can't 'stand-in', but they can do what you want, you just have to spell out each option. Something like this:
CASE [My Parameter]
WHEN 'Venue1' THEN IIF( ISNULL([venue1]) = False, "Keep", "Drop")
WHEN 'Venue2' THEN IIF( ISNULL([venue2]) = False, "Keep", "Drop")
WHEN 'Venue3' THEN IIF( ISNULL([venue3]) = False, "Keep", "Drop")
The IIF is a little easier to use than the IF in this case.
Is there a reason you have pivoted your data before connecting it to Tableau? YOu'll probably find it easier to have a dimension called "Venue" and will have more control over parameters that way.
Thanks Shawn. I was afraid that would be the answer. Thanks for the more efficient code!