I think the new actions on your junction model could work.
I’m using an action to update a date/time field when another field is changed on the same mode.
I have one action, which updates a field to the current date. There’s a specific option for that.
In your case, I think you’ll need a snippet, since your wanting to update one model field with a value from another model field.
I’ve seen samples around that do that, but I’m still new to Skuid so I can help with specifics. I’m just sure there’s lots of {} in it… 
Seth
