I can only think of one way to do this, but it could not be in the variability platform, but I think you can get the same results.
First, you will need an extra column in the data table, let's call it "Size" which has all the same values (say 1) EXCEPT for the rows where you want a larger marker. Let's give those rows a bigger number, say 10.
Create the graph in Graph Builder. You should be able to drag the x variables and nest them to get a similar look to what the Variability chart provides.
You can then drag the "Size" variable into the Size role, and it will draw the markers using that variable.
Here is a quick sample. Notice the two larger dots in Trudy's data:
Dan Obermiller