Right, you will not be able to do that.
Since the replace field in this case is a variable, that variable cannot contain any other expansion facilities, such as $1.
I have no solution for this. “Enhancing” the replace field to allow expansion characters within tokens would not be a good idea - tokens could already include any kind of characters and they are not necessarily intended to have meanings. For example, suppose I am trying to use a search & replace to change products to currency amounts, and the amount is in a variable as "$1.00", and that variable is used in the replacement. Suddenly that $1 has meaning where it should not.
So there basically isn't any way to support this.
You could, as an extreme, create the action from XML, after processing the variable to expand it, but there is no solution short of that that can be implemented safely.