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.