Success! Looks like we've fixed this one. According to our records the fix was applied for
a bug in our system
a recent build.
We made some changes to FieldLabel's innards in Beta 3. Specifically, calling setLabelSeparator (and a few other setters) immediately updates the label to reflect these changes. This should make it unnecessary to make the setText call after setLabelSeparator. Also, this should make it possible to use FieldLabel reliably in UiBinder.
This change has been included in Beta 3, so give it a try to see if this fixes your issue.
Yes, I can now set "labelSeparator" attribute in UiBinder and it works.
Empty value for "labelSeparator" also works.
The comment was for adding a new contructor or not. That is completely not UiBinder related.