The popup layout is easy.

It’s the application of the Mass Update based on what you’ve entered into popup that would be difficult.

Here’s how I would do it. 

1. Add a model of the same type as in the table to store the updates called “updateValues” or something.
2. Create a Mass Update action of Multiple Actions type
3. Cancel the model “updateValues”
4. Create a row in “updateValues”
5. Show a popup with  a field editor in edit mode for “updateValues”
6. Configure the field editor the way you’d like.
7. Add a Page Title in the popup to add Buttons.
8. The primary button would be something to apply the updates. This would almost certainly have to be a js snippet to apply the changes to the selected rows. You’d loop through the “Changes” in the “updateValues” model and then loop through the selected rows applying those changes.

Yup.

What’s the syntax to access the selected rows in the table once you’re in a different snippet (and therefore don’t get the arguments passed in)?