I am also trying to do something very similar to the original problem (with the very latest Community edition)
I have two fields on an single record Edit screen. When the user types something into the first text field ('foo' say) I want to capture this and change the value in the other field to be some corresponding default value ('bar' to keep with tradition!)
I am trying to do this by capturing the change with an OnChange action on the first field which then tries to set the value of the bound attribute variable of the second Web input widget (this is because you can't change the .Value of the widget (doesn't exist) - or the .TypedText value that is read only)
After setting the bound variable, I do an Ajax refresh of the second Web input widget and expect to see 'bar' as the widget should be updated with the value of the now changed bound attribute - But it shows nothing. I have traced the whole action and everything but the last ajax refresh step is working as expected.
I have changed the refesh to a full page Submit instead of Ajax and it all works - but it refreshes the whole form and that isn't what I wanted to do!