I think I have solved the issue! I am working on publishing it for the public.
Here is how you can do it:
You have to create the variable in the main workflow first, then re-create (tap on the variable name section until you see the little "Variables.." menu pop up) it on the main screen of the UI and change it's value (I haven't tried this with the textfield yet)
Now, in the main workflow, you can use the value of the variable as declared in the Custom UI action as if it is in the main workflow.
(see my "Test" link for an example for now)
It seems you have to have the variables outside of the custom UI action because the main workflow only recognizes "variable names" that are in the main workflow, however, the "values" of the variables, as declared in the Custom UI action, replace the values of the variables declared in he main workflow.
The only widget that can be used as a parameter in the main view of the UI is a widget that has prefilled information, because that information is there as soon as the UI pops up. Most widgets store information after they have been edited, , and the variable in the main UI is. It updated.
Correction, I have it working now with text fields, which change the variable to its input once the changes are committed.
Here is my Public Example