With a query, I populate a table with “real” (floating point) numbers. The number format attribute in the table customizer is working well to display the data the way I want. However, when I go to edit the data, then the number is displayed in all its floating point ugliness. How do I control the format of the number being edited?
Are you editing the number in the dataset using the dataset property of the table in the designer? If you are, unfortunately there is no way to change the format of the number when editing.
I am editing the value within the table (dataset) on a client (or in designer). 
The closest you can get to showing the pretty version of the value is to change the underlying query to format the float as a string. Then in your cellUpdate event, you can change the string back to a float and send that down to the DB.
Thank you.