If you have a gifted child with high stats who feels WUSTL is a good fit, but you are looking for strong merit aid, should such a child apply ED?
If not is there any other way to show interest in the school during RD?
I would not recommend applying ED in that situation. ED means you commit to the school if accepted. And WUSTL gives out very little merit aid. It actually is not based on high stats. There is a separate scholarship application and it is extremely competitive to get the few they offer. There is no other merit aid. That being said, I have two children who have been accepted there RD. Showing interest seems important. If your child can visit, definitely go. WashU does an excellent job showcasing the school. And there are lots of things to do in the area if you are far enough away that you want stay for a few days when you visit. In addition, if WashU visits your child’s school, have him/her go to the information session. They may not learn too much if they have already visited but it is another way to show interest. Applying for the scholarships is another way. I learned this year that once a student opens up the common app section for a particular school, the school is made aware of that. Have your child open up the common app portion for WashU early and complete it in advance of the deadline.
But, if your child absolutely needs merit aid for college, you should consider WashU a financial reach as the scholarships are very hard to come by. So make sure there are plenty of other schools on his/her list that are generous with merit aid. HTH
I am in a similar situation. I asked a visiting admissions rep how to show that WUSTL is my first choice, as financially I cannot commit to ED. She said to just send her a quick email when I submit my application that says WUSTL is my first choice school, and she would add it to my admissions file. You could do the same 
.