You can use a popover with a portal as long as it isn't placed inside the portal. But you can filter the list of items in a value list without needing to use a popover and there are ways to get auto-complete to work with them as well.
You could, for one example, select a category in a field outside of the portal and then the value list in your portal only lists items from that category.
Thanks. I wonder if it is possible to do this without using the portal at all? For instance, the portal might only display the related records, while some other button just below the portal could be labeled "Assign new Widget" and then it would show a popover where I could select the Widget, which would then assign it to the Customer, which would make it show up in the portal... I think the part I'm missing is how to a) create the row in the join table, and b) close the popover; once the widget record is clicked on... actually, I think your checkboxes demo in the ManyToManywDemoWExtras file could be adapted for that, I will try that.
You might also be interested in the examples found in this demo file: https://www.dropbox.com/s/0pm1gdqcfi2ndpv/EnhancedValueSelection.fp7
You'll need to open this using Open from FileMaker's File menu if you are using FileMaker 12 or 13 (as you are using if you are trying to set up popovers) in order to get a copy converted to the newer file format.
It includes an auto-complete enabled drop down list located outside of the portal which then adds a new row to a portal.